Import LLVM, at r72732.vendor/llvm/llvm-r72732

1 files changed, 251 insertions, 0 deletions

diff --git a/lib/Target/X86/README-X86-64.txt b/lib/Target/X86/README-X86-64.txt
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+//===- README_X86_64.txt - Notes for X86-64 code gen ----------------------===//
+
+Implement different PIC models? Right now we only support Mac OS X with small
+PIC code model.
+
+//===---------------------------------------------------------------------===//
+
+For this:
+
+extern void xx(void);
+void bar(void) {
+  xx();
+}
+
+gcc compiles to:
+
+.globl _bar
+_bar:
+	jmp	_xx
+
+We need to do the tailcall optimization as well.
+
+//===---------------------------------------------------------------------===//
+
+AMD64 Optimization Manual 8.2 has some nice information about optimizing integer
+multiplication by a constant. How much of it applies to Intel's X86-64
+implementation? There are definite trade-offs to consider: latency vs. register
+pressure vs. code size.
+
+//===---------------------------------------------------------------------===//
+
+Are we better off using branches instead of cmove to implement FP to
+unsigned i64?
+
+_conv:
+	ucomiss	LC0(%rip), %xmm0
+	cvttss2siq	%xmm0, %rdx
+	jb	L3
+	subss	LC0(%rip), %xmm0
+	movabsq	$-9223372036854775808, %rax
+	cvttss2siq	%xmm0, %rdx
+	xorq	%rax, %rdx
+L3:
+	movq	%rdx, %rax
+	ret
+
+instead of
+
+_conv:
+	movss LCPI1_0(%rip), %xmm1
+	cvttss2siq %xmm0, %rcx
+	movaps %xmm0, %xmm2
+	subss %xmm1, %xmm2
+	cvttss2siq %xmm2, %rax
+	movabsq $-9223372036854775808, %rdx
+	xorq %rdx, %rax
+	ucomiss %xmm1, %xmm0
+	cmovb %rcx, %rax
+	ret
+
+Seems like the jb branch has high likelyhood of being taken. It would have
+saved a few instructions.
+
+//===---------------------------------------------------------------------===//
+
+Poor codegen:
+
+int X[2];
+int b;
+void test(void) {
+  memset(X, b, 2*sizeof(X[0]));
+}
+
+llc:
+	movq _b@GOTPCREL(%rip), %rax
+	movzbq (%rax), %rax
+	movq %rax, %rcx
+	shlq $8, %rcx
+	orq %rax, %rcx
+	movq %rcx, %rax
+	shlq $16, %rax
+	orq %rcx, %rax
+	movq %rax, %rcx
+	shlq $32, %rcx
+	movq _X@GOTPCREL(%rip), %rdx
+	orq %rax, %rcx
+	movq %rcx, (%rdx)
+	ret
+
+gcc:
+	movq	_b@GOTPCREL(%rip), %rax
+	movabsq	$72340172838076673, %rdx
+	movzbq	(%rax), %rax
+	imulq	%rdx, %rax
+	movq	_X@GOTPCREL(%rip), %rdx
+	movq	%rax, (%rdx)
+	ret
+
+//===---------------------------------------------------------------------===//
+
+Vararg function prologue can be further optimized. Currently all XMM registers
+are stored into register save area. Most of them can be eliminated since the
+upper bound of the number of XMM registers used are passed in %al. gcc produces
+something like the following:
+
+	movzbl	%al, %edx
+	leaq	0(,%rdx,4), %rax
+	leaq	4+L2(%rip), %rdx
+	leaq	239(%rsp), %rax
+       	jmp	*%rdx
+	movaps	%xmm7, -15(%rax)
+	movaps	%xmm6, -31(%rax)
+	movaps	%xmm5, -47(%rax)
+	movaps	%xmm4, -63(%rax)
+	movaps	%xmm3, -79(%rax)
+	movaps	%xmm2, -95(%rax)
+	movaps	%xmm1, -111(%rax)
+	movaps	%xmm0, -127(%rax)
+L2:
+
+It jumps over the movaps that do not need to be stored. Hard to see this being
+significant as it added 5 instruciton (including a indirect branch) to avoid
+executing 0 to 8 stores in the function prologue.
+
+Perhaps we can optimize for the common case where no XMM registers are used for
+parameter passing. i.e. is %al == 0 jump over all stores. Or in the case of a
+leaf function where we can determine that no XMM input parameter is need, avoid
+emitting the stores at all.
+
+//===---------------------------------------------------------------------===//
+
+AMD64 has a complex calling convention for aggregate passing by value:
+
+1. If the size of an object is larger than two eightbytes, or in C++, is a non- 
+   POD structure or union type, or contains unaligned fields, it has class 
+   MEMORY.
+2. Both eightbytes get initialized to class NO_CLASS. 
+3. Each field of an object is classified recursively so that always two fields
+   are considered. The resulting class is calculated according to the classes
+   of the fields in the eightbyte: 
+   (a) If both classes are equal, this is the resulting class. 
+   (b) If one of the classes is NO_CLASS, the resulting class is the other 
+       class. 
+   (c) If one of the classes is MEMORY, the result is the MEMORY class. 
+   (d) If one of the classes is INTEGER, the result is the INTEGER. 
+   (e) If one of the classes is X87, X87UP, COMPLEX_X87 class, MEMORY is used as
+      class. 
+   (f) Otherwise class SSE is used. 
+4. Then a post merger cleanup is done: 
+   (a) If one of the classes is MEMORY, the whole argument is passed in memory. 
+   (b) If SSEUP is not preceeded by SSE, it is converted to SSE.
+
+Currently llvm frontend does not handle this correctly.
+
+Problem 1:
+    typedef struct { int i; double d; } QuadWordS;
+It is currently passed in two i64 integer registers. However, gcc compiled
+callee expects the second element 'd' to be passed in XMM0.
+
+Problem 2:
+    typedef struct { int32_t i; float j; double d; } QuadWordS;
+The size of the first two fields == i64 so they will be combined and passed in
+a integer register RDI. The third field is still passed in XMM0.
+
+Problem 3:
+    typedef struct { int64_t i; int8_t j; int64_t d; } S;
+    void test(S s)
+The size of this aggregate is greater than two i64 so it should be passed in 
+memory. Currently llvm breaks this down and passed it in three integer
+registers.
+
+Problem 4:
+Taking problem 3 one step ahead where a function expects a aggregate value
+in memory followed by more parameter(s) passed in register(s).
+    void test(S s, int b)
+
+LLVM IR does not allow parameter passing by aggregates, therefore it must break
+the aggregates value (in problem 3 and 4) into a number of scalar values:
+    void %test(long %s.i, byte %s.j, long %s.d);
+
+However, if the backend were to lower this code literally it would pass the 3
+values in integer registers. To force it be passed in memory, the frontend
+should change the function signiture to:
+    void %test(long %undef1, long %undef2, long %undef3, long %undef4, 
+               long %undef5, long %undef6,
+               long %s.i, byte %s.j, long %s.d);
+And the callee would look something like this:
+    call void %test( undef, undef, undef, undef, undef, undef,
+                     %tmp.s.i, %tmp.s.j, %tmp.s.d );
+The first 6 undef parameters would exhaust the 6 integer registers used for
+parameter passing. The following three integer values would then be forced into
+memory.
+
+For problem 4, the parameter 'd' would be moved to the front of the parameter
+list so it will be passed in register:
+    void %test(int %d,
+               long %undef1, long %undef2, long %undef3, long %undef4, 
+               long %undef5, long %undef6,
+               long %s.i, byte %s.j, long %s.d);
+
+//===---------------------------------------------------------------------===//
+
+Right now the asm printer assumes GlobalAddress are accessed via RIP relative
+addressing. Therefore, it is not possible to generate this:
+        movabsq $__ZTV10polynomialIdE+16, %rax
+
+That is ok for now since we currently only support small model. So the above
+is selected as
+        leaq __ZTV10polynomialIdE+16(%rip), %rax
+
+This is probably slightly slower but is much shorter than movabsq. However, if
+we were to support medium or larger code models, we need to use the movabs
+instruction. We should probably introduce something like AbsoluteAddress to
+distinguish it from GlobalAddress so the asm printer and JIT code emitter can
+do the right thing.
+
+//===---------------------------------------------------------------------===//
+
+It's not possible to reference AH, BH, CH, and DH registers in an instruction
+requiring REX prefix. However, divb and mulb both produce results in AH. If isel
+emits a CopyFromReg which gets turned into a movb and that can be allocated a
+r8b - r15b.
+
+To get around this, isel emits a CopyFromReg from AX and then right shift it
+down by 8 and truncate it. It's not pretty but it works. We need some register
+allocation magic to make the hack go away (e.g. putting additional constraints
+on the result of the movb).
+
+//===---------------------------------------------------------------------===//
+
+The x86-64 ABI for hidden-argument struct returns requires that the
+incoming value of %rdi be copied into %rax by the callee upon return.
+
+The idea is that it saves callers from having to remember this value,
+which would often require a callee-saved register. Callees usually
+need to keep this value live for most of their body anyway, so it
+doesn't add a significant burden on them.
+
+We currently implement this in codegen, however this is suboptimal
+because it means that it would be quite awkward to implement the
+optimization for callers.
+
+A better implementation would be to relax the LLVM IR rules for sret
+arguments to allow a function with an sret argument to have a non-void
+return type, and to have the front-end to set up the sret argument value
+as the return value of the function. The front-end could more easily
+emit uses of the returned struct value to be in terms of the function's
+lowered return value, and it would free non-C frontends from a
+complication only required by a C-based ABI.
+
+//===---------------------------------------------------------------------===//