aboutsummaryrefslogtreecommitdiff
path: root/include/lldb/Expression/IRForTarget.h
blob: 502f796d15a112ef1db9a00a206bcfb4c607a649 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
//===-- IRForTarget.h ---------------------------------------------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#ifndef liblldb_IRForTarget_h_
#define liblldb_IRForTarget_h_

#include "lldb/lldb-public.h"
#include "lldb/Core/ConstString.h"
#include "lldb/Core/Error.h"
#include "lldb/Core/Stream.h"
#include "lldb/Core/StreamString.h"
#include "lldb/Symbol/TaggedASTType.h"
#include "llvm/Pass.h"

#include <map>
#include <functional>

namespace llvm {
    class BasicBlock;
    class CallInst;
    class Constant;
    class ConstantInt;
    class Function;
    class GlobalValue;
    class GlobalVariable;
    class Instruction;
    class IntegerType;
    class Module;
    class StoreInst;
    class DataLayout;
    class Type;
    class Value;
}

namespace lldb_private {
    class ClangExpressionDeclMap;
    class IRExecutionUnit;
    class IRMemoryMap;
}

//----------------------------------------------------------------------
/// @class IRForTarget IRForTarget.h "lldb/Expression/IRForTarget.h"
/// @brief Transforms the IR for a function to run in the target
///
/// Once an expression has been parsed and converted to IR, it can run
/// in two contexts: interpreted by LLDB as a DWARF location expression,
/// or compiled by the JIT and inserted into the target process for
/// execution.
///
/// IRForTarget makes the second possible, by applying a series of
/// transformations to the IR which make it relocatable.  These
/// transformations are discussed in more detail next to their relevant
/// functions.
//----------------------------------------------------------------------
class IRForTarget : public llvm::ModulePass
{
public:
    //------------------------------------------------------------------
    /// Constructor
    ///
    /// @param[in] decl_map
    ///     The list of externally-referenced variables for the expression,
    ///     for use in looking up globals and allocating the argument
    ///     struct.  See the documentation for ClangExpressionDeclMap.
    ///
    /// @param[in] resolve_vars
    ///     True if the external variable references (including persistent
    ///     variables) should be resolved.  If not, only external functions
    ///     are resolved.
    ///
    /// @param[in] execution_policy
    ///     Determines whether an IR interpreter can be used to statically
    ///     evaluate the expression.
    ///
    /// @param[in] const_result
    ///     This variable is populated with the statically-computed result
    ///     of the function, if it has no side-effects and the result can
    ///     be computed statically.
    ///
    /// @param[in] execution_unit
    ///     The holder for raw data associated with the expression.
    ///
    /// @param[in] error_stream
    ///     If non-NULL, a stream on which errors can be printed.
    ///
    /// @param[in] func_name
    ///     The name of the function to prepare for execution in the target.
    //------------------------------------------------------------------
    IRForTarget(lldb_private::ClangExpressionDeclMap *decl_map,
                bool resolve_vars,
                lldb_private::IRExecutionUnit &execution_unit,
                lldb_private::Stream *error_stream,
                const char* func_name = "$__lldb_expr");
    
    //------------------------------------------------------------------
    /// Destructor
    //------------------------------------------------------------------
    virtual ~IRForTarget();
    
    //------------------------------------------------------------------
    /// Run this IR transformer on a single module
    ///
    /// Implementation of the llvm::ModulePass::runOnModule() function.
    ///
    /// @param[in] llvm_module
    ///     The module to run on.  This module is searched for the function
    ///     $__lldb_expr, and that function is passed to the passes one by 
    ///     one.
    ///
    /// @param[in] interpreter_error
    ///     An error.  If the expression fails to be interpreted, this error
    ///     is set to a reason why.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    virtual bool 
    runOnModule (llvm::Module &llvm_module);
    
    //------------------------------------------------------------------
    /// Interface stub
    ///
    /// Implementation of the llvm::ModulePass::assignPassManager() 
    /// function.
    //------------------------------------------------------------------
    virtual void
    assignPassManager (llvm::PMStack &pass_mgr_stack,
                       llvm::PassManagerType pass_mgr_type = llvm::PMT_ModulePassManager);
    
    //------------------------------------------------------------------
    /// Returns PMT_ModulePassManager
    ///
    /// Implementation of the llvm::ModulePass::getPotentialPassManagerType() 
    /// function.
    //------------------------------------------------------------------
    virtual llvm::PassManagerType 
    getPotentialPassManagerType() const;

private:
    //------------------------------------------------------------------
    /// Ensures that the current function's linkage is set to external.
    /// Otherwise the JIT may not return an address for it.
    ///
    /// @param[in] llvm_function
    ///     The function whose linkage is to be fixed.
    ///
    /// @return
    ///     True on success; false otherwise.
    //------------------------------------------------------------------
    bool 
    FixFunctionLinkage (llvm::Function &llvm_function);
    
    //------------------------------------------------------------------
    /// A module-level pass to replace all function pointers with their
    /// integer equivalents.
    //------------------------------------------------------------------
    
    //------------------------------------------------------------------
    /// The top-level pass implementation
    ///
    /// @param[in] llvm_module
    ///     The module currently being processed.
    ///
    /// @param[in] llvm_function
    ///     The function currently being processed.
    ///
    /// @return
    ///     True on success; false otherwise.
    //------------------------------------------------------------------
    bool 
    HasSideEffects (llvm::Function &llvm_function);
    
    //------------------------------------------------------------------
    /// A function-level pass to check whether the function has side
    /// effects.
    //------------------------------------------------------------------
    
    //------------------------------------------------------------------
    /// Get the address of a fuction, and a location to put the complete
    /// Value of the function if one is available.
    ///
    /// @param[in] function
    ///     The function to find the location of.
    ///
    /// @param[out] ptr
    ///     The location of the function in the target.
    ///
    /// @param[out] name
    ///     The resolved name of the function (matters for intrinsics).
    ///
    /// @param[out] value_ptr
    ///     A variable to put the function's completed Value* in, or NULL
    ///     if the Value* shouldn't be stored anywhere.
    ///
    /// @return
    ///     The pointer.
    //------------------------------------------------------------------ 
    bool 
    GetFunctionAddress (llvm::Function *function,
                        uint64_t &ptr,
                        lldb_private::ConstString &name,
                        llvm::Constant **&value_ptr);
    
    //------------------------------------------------------------------
    /// Build a function pointer given a type and a raw pointer.
    ///
    /// @param[in] type
    ///     The type of the function pointer to be built.
    ///
    /// @param[in] ptr
    ///     The value of the pointer.
    ///
    /// @return
    ///     The pointer.
    //------------------------------------------------------------------ 
    llvm::Constant *
    BuildFunctionPointer (llvm::Type *type,
                          uint64_t ptr);
    
    void
    RegisterFunctionMetadata (llvm::LLVMContext &context,
                              llvm::Value *function_ptr,
                              const char *name);
    
    //------------------------------------------------------------------
    /// The top-level pass implementation
    ///
    /// @param[in] llvm_function
    ///     The function currently being processed.
    ///
    /// @return
    ///     True if the function has side effects (or if this cannot
    ///     be determined); false otherwise.
    //------------------------------------------------------------------
    bool 
    ResolveFunctionPointers (llvm::Module &llvm_module);
    
    //------------------------------------------------------------------
    /// A function-level pass to take the generated global value
    /// $__lldb_expr_result and make it into a persistent variable.
    /// Also see ASTResultSynthesizer.
    //------------------------------------------------------------------
    
    //------------------------------------------------------------------
    /// Find the NamedDecl corresponding to a Value.  This interface is
    /// exposed for the IR interpreter.
    ///
    /// @param[in] module
    ///     The module containing metadata to search
    ///
    /// @param[in] global
    ///     The global entity to search for
    ///
    /// @return
    ///     The corresponding variable declaration
    //------------------------------------------------------------------
public:
    static clang::NamedDecl *
    DeclForGlobal (const llvm::GlobalValue *global_val, llvm::Module *module);
private:
    clang::NamedDecl *
    DeclForGlobal (llvm::GlobalValue *global);
    
    //------------------------------------------------------------------
    /// Set the constant result variable m_const_result to the provided
    /// constant, assuming it can be evaluated.  The result variable
    /// will be reset to NULL later if the expression has side effects.
    ///
    /// @param[in] initializer
    ///     The constant initializer for the variable.
    ///
    /// @param[in] name
    ///     The name of the result variable.
    ///
    /// @param[in] type
    ///     The Clang type of the result variable.
    //------------------------------------------------------------------    
    void 
    MaybeSetConstantResult (llvm::Constant *initializer,
                            const lldb_private::ConstString &name,
                            lldb_private::TypeFromParser type);
    
    //------------------------------------------------------------------
    /// If the IR represents a cast of a variable, set m_const_result
    /// to the result of the cast.  The result variable will be reset to
    /// NULL latger if the expression has side effects.
    ///
    /// @param[in] type
    ///     The Clang type of the result variable.
    //------------------------------------------------------------------  
    void
    MaybeSetCastResult (lldb_private::TypeFromParser type);
    
    //------------------------------------------------------------------
    /// The top-level pass implementation
    ///
    /// @param[in] llvm_function
    ///     The function currently being processed.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool 
    CreateResultVariable (llvm::Function &llvm_function);
    
    //------------------------------------------------------------------
    /// A module-level pass to find Objective-C constant strings and
    /// transform them to calls to CFStringCreateWithBytes.
    //------------------------------------------------------------------

    //------------------------------------------------------------------
    /// Rewrite a single Objective-C constant string.
    ///
    /// @param[in] NSStr
    ///     The constant NSString to be transformed
    ///
    /// @param[in] CStr
    ///     The constant C string inside the NSString.  This will be
    ///     passed as the bytes argument to CFStringCreateWithBytes.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool 
    RewriteObjCConstString (llvm::GlobalVariable *NSStr,
                            llvm::GlobalVariable *CStr);    
    
    //------------------------------------------------------------------
    /// The top-level pass implementation
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool 
    RewriteObjCConstStrings ();

    //------------------------------------------------------------------
    /// A basic block-level pass to find all Objective-C method calls and
    /// rewrite them to use sel_registerName instead of statically allocated
    /// selectors.  The reason is that the selectors are created on the
    /// assumption that the Objective-C runtime will scan the appropriate
    /// section and prepare them.  This doesn't happen when code is copied
    /// into the target, though, and there's no easy way to induce the
    /// runtime to scan them.  So instead we get our selectors from
    /// sel_registerName.
    //------------------------------------------------------------------

    //------------------------------------------------------------------
    /// Replace a single selector reference
    ///
    /// @param[in] selector_load
    ///     The load of the statically-allocated selector.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool 
    RewriteObjCSelector (llvm::Instruction* selector_load);
    
    //------------------------------------------------------------------
    /// The top-level pass implementation
    ///
    /// @param[in] basic_block
    ///     The basic block currently being processed.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool 
    RewriteObjCSelectors (llvm::BasicBlock &basic_block);
    
    //------------------------------------------------------------------
    /// A basic block-level pass to find all newly-declared persistent
    /// variables and register them with the ClangExprDeclMap.  This 
    /// allows them to be materialized and dematerialized like normal
    /// external variables.  Before transformation, these persistent
    /// variables look like normal locals, so they have an allocation.
    /// This pass excises these allocations and makes references look
    /// like external references where they will be resolved -- like all
    /// other external references -- by ResolveExternals().
    //------------------------------------------------------------------

    //------------------------------------------------------------------
    /// Handle a single allocation of a persistent variable
    ///
    /// @param[in] persistent_alloc
    ///     The allocation of the persistent variable.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool 
    RewritePersistentAlloc (llvm::Instruction *persistent_alloc);
    
    //------------------------------------------------------------------
    /// The top-level pass implementation
    ///
    /// @param[in] basic_block
    ///     The basic block currently being processed.
    //------------------------------------------------------------------
    bool 
    RewritePersistentAllocs (llvm::BasicBlock &basic_block);
    
    //------------------------------------------------------------------
    /// A function-level pass to find all external variables and functions 
    /// used in the IR.  Each found external variable is added to the 
    /// struct, and each external function is resolved in place, its call
    /// replaced with a call to a function pointer whose value is the 
    /// address of the function in the target process.
    //------------------------------------------------------------------
    
    //------------------------------------------------------------------
    /// Write an initializer to a memory array of assumed sufficient
    /// size.
    ///
    /// @param[in] data
    ///     A pointer to the data to write to.
    ///
    /// @param[in] initializer
    ///     The initializer itself.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool
    MaterializeInitializer (uint8_t *data, llvm::Constant *initializer);
    
    //------------------------------------------------------------------
    /// Move an internal variable into the static allocation section.
    ///
    /// @param[in] global_variable
    ///     The variable.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool
    MaterializeInternalVariable (llvm::GlobalVariable *global_variable);
    
    //------------------------------------------------------------------
    /// Handle a single externally-defined variable
    ///
    /// @param[in] value
    ///     The variable.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool 
    MaybeHandleVariable (llvm::Value *value);
    
    //------------------------------------------------------------------
    /// Handle a single externally-defined symbol
    ///
    /// @param[in] symbol
    ///     The symbol.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool
    HandleSymbol (llvm::Value *symbol);
    
    //------------------------------------------------------------------
    /// Handle a single externally-defined Objective-C class
    ///
    /// @param[in] classlist_reference
    ///     The reference, usually "01L_OBJC_CLASSLIST_REFERENCES_$_n"
    ///     where n (if present) is an index.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool
    HandleObjCClass(llvm::Value *classlist_reference);

    //------------------------------------------------------------------
    /// Handle all the arguments to a function call
    ///
    /// @param[in] C
    ///     The call instruction.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool 
    MaybeHandleCallArguments (llvm::CallInst *call_inst);
    
    //------------------------------------------------------------------
    /// Resolve variable references in calls to external functions
    ///
    /// @param[in] basic_block
    ///     The basic block currently being processed.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool 
    ResolveCalls (llvm::BasicBlock &basic_block);
    
    //------------------------------------------------------------------
    /// Remove calls to __cxa_atexit, which should never be generated by
    /// expressions.
    ///
    /// @param[in] call_inst
    ///     The call instruction.
    ///
    /// @return
    ///     True if the scan was successful; false if some operation
    ///     failed
    //------------------------------------------------------------------
    bool
    RemoveCXAAtExit (llvm::BasicBlock &basic_block);
    
    //------------------------------------------------------------------
    /// The top-level pass implementation
    ///
    /// @param[in] basic_block
    ///     The function currently being processed.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool 
    ResolveExternals (llvm::Function &llvm_function);
    
    //------------------------------------------------------------------
    /// A basic block-level pass to excise guard variables from the code.
    /// The result for the function is passed through Clang as a static
    /// variable.  Static variables normally have guard variables to
    /// ensure that they are only initialized once.  
    //------------------------------------------------------------------
    
    //------------------------------------------------------------------
    /// Rewrite a load to a guard variable to return constant 0.
    ///
    /// @param[in] guard_load
    ///     The load instruction to zero out.
    //------------------------------------------------------------------
    void
    TurnGuardLoadIntoZero(llvm::Instruction* guard_load);
    
    //------------------------------------------------------------------
    /// The top-level pass implementation
    ///
    /// @param[in] basic_block
    ///     The basic block currently being processed.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool 
    RemoveGuards (llvm::BasicBlock &basic_block);
    
    //------------------------------------------------------------------
    /// A module-level pass to allocate all string literals in a separate
    /// allocation and redirect references to them.
    //------------------------------------------------------------------
    
    //------------------------------------------------------------------
    /// The top-level pass implementation
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool 
    ReplaceStrings ();
    
    //------------------------------------------------------------------
    /// A basick block-level pass to find all literals that will be 
    /// allocated as statics by the JIT (in contrast to the Strings, 
    /// which already are statics) and synthesize loads for them.
    //------------------------------------------------------------------
    
    //------------------------------------------------------------------
    /// The top-level pass implementation
    ///
    /// @param[in] basic_block
    ///     The basic block currently being processed.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool 
    ReplaceStaticLiterals (llvm::BasicBlock &basic_block);
    
    //------------------------------------------------------------------
    /// A function-level pass to make all external variable references
    /// point at the correct offsets from the void* passed into the
    /// function.  ClangExpressionDeclMap::DoStructLayout() must be called
    /// beforehand, so that the offsets are valid.
    //------------------------------------------------------------------
    
    //------------------------------------------------------------------
    /// The top-level pass implementation
    ///
    /// @param[in] llvm_function
    ///     The function currently being processed.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool 
    ReplaceVariables (llvm::Function &llvm_function);
    
    //------------------------------------------------------------------
    /// A module-level pass to remove all global variables from the
    /// module since it no longer should export or import any symbols.
    //------------------------------------------------------------------
    
    //------------------------------------------------------------------
    /// The top-level pass implementation
    ///
    /// @param[in] llvm_module
    ///     The module currently being processed.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool
    StripAllGVs (llvm::Module &llvm_module);
    
    class StaticDataAllocator {
    public:
        StaticDataAllocator(lldb_private::IRExecutionUnit &execution_unit);
        lldb_private::StreamString &GetStream()
        {
            return m_stream_string;
        }
        lldb::addr_t Allocate();
    private:
        lldb_private::IRExecutionUnit  &m_execution_unit;
        lldb_private::StreamString      m_stream_string;
        lldb::addr_t                    m_allocation;
    };
    
    /// Flags
    bool                                    m_resolve_vars;             ///< True if external variable references and persistent variable references should be resolved
    std::string                             m_func_name;                ///< The name of the function to translate
    lldb_private::ConstString               m_result_name;              ///< The name of the result variable ($0, $1, ...)
    lldb_private::TypeFromParser            m_result_type;              ///< The type of the result variable.
    llvm::Module                           *m_module;                   ///< The module being processed, or NULL if that has not been determined yet.
    std::unique_ptr<llvm::DataLayout>       m_target_data;              ///< The target data for the module being processed, or NULL if there is no module.
    lldb_private::ClangExpressionDeclMap   *m_decl_map;                 ///< The DeclMap containing the Decls 
    StaticDataAllocator                     m_data_allocator;           ///< The allocator to use for constant strings
    llvm::Constant                         *m_CFStringCreateWithBytes;  ///< The address of the function CFStringCreateWithBytes, cast to the appropriate function pointer type
    llvm::Constant                         *m_sel_registerName;         ///< The address of the function sel_registerName, cast to the appropriate function pointer type
    llvm::IntegerType                      *m_intptr_ty;                ///< The type of an integer large enough to hold a pointer.
    lldb_private::Stream                   *m_error_stream;             ///< If non-NULL, the stream on which errors should be printed
    
    llvm::StoreInst                        *m_result_store;             ///< If non-NULL, the store instruction that writes to the result variable.  If m_has_side_effects is true, this is NULL.
    bool                                    m_result_is_pointer;        ///< True if the function's result in the AST is a pointer (see comments in ASTResultSynthesizer::SynthesizeBodyResult)
    
    llvm::GlobalVariable                   *m_reloc_placeholder;        ///< A placeholder that will be replaced by a pointer to the final location of the static allocation.
    
    //------------------------------------------------------------------
    /// UnfoldConstant operates on a constant [Old] which has just been 
    /// replaced with a value [New].  We assume that new_value has 
    /// been properly placed early in the function, in front of the 
    /// first instruction in the entry basic block 
    /// [FirstEntryInstruction].  
    ///
    /// UnfoldConstant reads through the uses of Old and replaces Old 
    /// in those uses with New.  Where those uses are constants, the 
    /// function generates new instructions to compute the result of the 
    /// new, non-constant expression and places them before 
    /// FirstEntryInstruction.  These instructions replace the constant
    /// uses, so UnfoldConstant calls itself recursively for those.
    ///
    /// @param[in] llvm_function
    ///     The function currently being processed.
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    
    class FunctionValueCache {
    public:
        typedef std::function <llvm::Value *(llvm::Function *)> Maker;

        FunctionValueCache (Maker const &maker);
        ~FunctionValueCache ();
        llvm::Value *GetValue (llvm::Function *function);
    private:
        Maker const m_maker;
        typedef std::map<llvm::Function *, llvm::Value *> FunctionValueMap;
        FunctionValueMap m_values;
    };
    
    FunctionValueCache m_entry_instruction_finder;
    
    static bool
    UnfoldConstant (llvm::Constant *old_constant, 
                    FunctionValueCache &value_maker,
                    FunctionValueCache &entry_instruction_finder);
    
    //------------------------------------------------------------------
    /// Construct a reference to m_reloc_placeholder with a given type
    /// and offset.  This typically happens after inserting data into
    /// m_data_allocator.
    ///
    /// @param[in] type
    ///     The type of the value being loaded.
    ///
    /// @param[in] offset
    ///     The offset of the value from the base of m_data_allocator.
    ///
    /// @return
    ///     The Constant for the reference, usually a ConstantExpr.
    //------------------------------------------------------------------
    llvm::Constant *
    BuildRelocation(llvm::Type *type, 
                    uint64_t offset);
    
    //------------------------------------------------------------------
    /// Commit the allocation in m_data_allocator and use its final
    /// location to replace m_reloc_placeholder.
    ///
    /// @param[in] module
    ///     The module that m_data_allocator resides in
    ///
    /// @return
    ///     True on success; false otherwise
    //------------------------------------------------------------------
    bool 
    CompleteDataAllocation ();

};

#endif