amd64: move fusufault after all users

A lot of function have the following check:
        cmpq    %rax,%rdi                       /* verify address is valid */
        ja      fusufault

The label is present earlier in kernel .text, which means this is a jump
backwards. Absent any information in branch predictor, the cpu predicts it
as taken. Since it is almost never taken in practice, this results in a
completely avoidable misprediction.

Move it past all consumers, so that it is predicted as not taken.

Approved by:	re (kib)

1 files changed, 10 insertions, 10 deletions

diff --git a/sys/amd64/amd64/support.S b/sys/amd64/amd64/support.S
index bc849b21d965..33ffec1ff5f9 100644
--- a/sys/amd64/amd64/support.S
+++ b/sys/amd64/amd64/support.S
@@ -755,16 +755,6 @@ ENTRY(fubyte_smap)
 	ret
 END(fubyte_smap)
 
-	ALIGN_TEXT
-	/* Fault entry clears PSL.AC */
-fusufault:
-	movq	PCPU(CURPCB),%rcx
-	xorl	%eax,%eax
-	movq	%rax,PCB_ONFAULT(%rcx)
-	decq	%rax
-	POP_FRAME_POINTER
-	ret
-
 /*
  * Store a 64-bit word, a 32-bit word, a 16-bit word, or an 8-bit byte to
  * user memory.
@@ -916,6 +906,16 @@ ENTRY(subyte_smap)
 	ret
 END(subyte_smap)
 
+	ALIGN_TEXT
+	/* Fault entry clears PSL.AC */
+fusufault:
+	movq	PCPU(CURPCB),%rcx
+	xorl	%eax,%eax
+	movq	%rax,PCB_ONFAULT(%rcx)
+	decq	%rax
+	POP_FRAME_POINTER
+	ret
+
 /*
  * copyinstr(from, to, maxlen, int *lencopied)
  *           %rdi, %rsi, %rdx, %rcx