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A few more small things I've run into.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47702 91177308-0d34-0410-b5e6-96231b3b80d8
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Eli Friedman 2008-02-28 00:21:43 +00:00
parent 7b1b7f5c5a
commit 41ce5b82da
1 changed files with 86 additions and 3 deletions
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89
lib/Target/X86/README.txt
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@ -194,9 +194,9 @@ when we can spare a register. It reduces code size.
Evaluate what the best way to codegen sdiv X, (2^C) is. For X/8, we currently
get this:
int %test1(int %X) {
%Y = div int %X, 8
ret int %Y
define i32 @test1(i32 %X) {
%Y = sdiv i32 %X, 8
ret i32 %Y
}
_test1:
@ -1604,3 +1604,86 @@ can optimize this specific case even more to:
sbbl %ecx, %ecx
//===---------------------------------------------------------------------===//
Take the following code (from
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16541):
extern unsigned char first_one[65536];
int FirstOnet(unsigned long long arg1)
{
if (arg1 >> 48)
return (first_one[arg1 >> 48]);
return 0;
}
The following code is currently generated:
FirstOnet:
movl 8(%esp), %eax
cmpl $65536, %eax
movl 4(%esp), %ecx
jb .LBB1_2 # UnifiedReturnBlock
.LBB1_1: # ifthen
shrl $16, %eax
movzbl first_one(%eax), %eax
ret
.LBB1_2: # UnifiedReturnBlock
xorl %eax, %eax
ret
There are a few possible improvements here:
1. We should be able to eliminate the dead load into %ecx
2. We could change the "movl 8(%esp), %eax" into
"movzwl 10(%esp), %eax"; this lets us change the cmpl
into a testl, which is shorter, and eliminate the shift.
We could also in theory eliminate the branch by using a conditional
for the address of the load, but that seems unlikely to be worthwhile
in general.
//===---------------------------------------------------------------------===//
Take the following code:
#include <xmmintrin.h>
__m128i doload64(short x) {return _mm_set_epi16(x,x,x,x,x,x,x,x);}
LLVM currently generates the following on x86:
doload64:
movzwl 4(%esp), %eax
movd %eax, %xmm0
punpcklwd %xmm0, %xmm0
pshufd $0, %xmm0, %xmm0
ret
gcc's generated code:
doload64:
movd 4(%esp), %xmm0
punpcklwd %xmm0, %xmm0
pshufd $0, %xmm0, %xmm0
ret
LLVM should be able to generate the same thing as gcc.
//===---------------------------------------------------------------------===//
Take the following code:
#include <xmmintrin.h>
__m128i doload64(short x) {return _mm_set_epi16(0,0,0,0,0,0,0,1);}
On x86, LLVM generates the following:
doload64:
subl $28, %esp
movl $0, 4(%esp)
movl $1, (%esp)
movq (%esp), %xmm0
addl $28, %esp
ret
LLVM should instead generate something more like the following:
doload64:
movl $1, %eax
movd %eax, %xmm0
ret
//===---------------------------------------------------------------------===//