It did some changes to CompareByte in rtl/x86_64/x86_64.inc to reduce
the code size and make it run faster (see attached path). I was
successful with the code size deduction (47 bytes vs. 62 bytes) and also
with the speed (according to a micro benchmark [1] run on an Ivy Bridge
desktop).
To achieve this I used movzbl twice. But then I came across the comment
in FillChar (also in rtl/x86_64/x86_64.inc) about movzbl breaking
targets using external GAS (Mantis #19188). As this Mantis issue is
dated back in 2011 my question is: Is this still valid? And what would
be the preferred way to overcome this issue?
{$ifdef oldbinutils}
.byte 0x0F,0xb6,0x01
{$else}
movzbl (%rcx),%eax
{$endif}
Markus
[1] the benchmark compares a 10 MB memory block with itself 10000 times
Index: trunk/rtl/x86_64/x86_64.inc
===================================================================
--- trunk/rtl/x86_64/x86_64.inc (Revision 37365)
+++ trunk/rtl/x86_64/x86_64.inc (Arbeitskopie)
@@ -645,8 +645,8 @@
.balign 8
.LCmpbyteLoop:
- movb (%rcx),%r9b
- cmpb (%rdx),%r9b
+ movzbl (%rcx),%eax
+ cmpb (%rdx),%al
leaq 1(%rcx),%rcx
leaq 1(%rdx),%rdx
jne .LCmpbyteExitFast
@@ -653,14 +653,12 @@
decq %r8
jne .LCmpbyteLoop
.LCmpbyteExitFast:
- movzbq -1(%rdx),%r8 { Compare last position }
- movzbq %r9b,%rax
- subq %r8,%rax
- ret
+ movzbl -1(%rdx),%ecx { Compare last position }
+ subq %rcx,%rax
+ retq
.LCmpbyteZero:
- movq $0,%rax
- ret
+ xorl %eax,%eax
end;
{$endif FPC_SYSTEM_HAS_COMPAREBYTE}
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