On Dec 5, 2010, at 3:19 AM, Richard Guenther wrote:
>> $ clang t.cc -S -o - -O3 -mkernel -fomit-frame-pointer -mllvm
>> -show-mc-encoding
>> .section __TEXT,__text,regular,pure_instructions
>> .globl __Z4testl
>> .align 4, 0x90
>> __Z4testl: ## @_Z4testl
>> ## BB#0: ## %entry
>> movl $4, %ecx ## encoding:
>> [0xb9,0x04,0x00,0x00,0x00]
>> movq %rdi, %rax ## encoding: [0x48,0x89,0xf8]
>> mulq %rcx ## encoding: [0x48,0xf7,0xe1]
>> movq $-1, %rdi ## encoding:
>> [0x48,0xc7,0xc7,0xff,0xff,0xff,0xff]
>> cmovnoq %rax, %rdi ## encoding: [0x48,0x0f,0x41,0xf8]
>> jmp __Znam ## TAILCALL
>> ## encoding: [0xeb,A]
>> ## fixup A - offset: 1, value:
>> __Znam-1, kind: FK_PCRel_1
>> .subsections_via_symbols
>>
>> This could be further improved by inverting the cmov condition to avoid the
>> first movq, which we'll tackle as a general regalloc improvement.
>
> I'm curious as on how you represent the overflow checking in your highlevel
> IL.
The (optimized) generated IR is:
$ clang t.cc -emit-llvm -S -o - -O3
...
define noalias i8* @_Z4testl(i64 %count) ssp {
entry:
%0 = tail call %0 @llvm.umul.with.overflow.i64(i64 %count, i64 4)
%1 = extractvalue %0 %0, 1
%2 = extractvalue %0 %0, 0
%3 = select i1 %1, i64 -1, i64 %2
%call = tail call noalias i8* @_Znam(i64 %3)
ret i8* %call
}
More information on the overflow intrinsics is here:
http://llvm.org/docs/LangRef.html#int_overflow
-Chris
