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1278
2015 Apr 20
3
[LLVMdev] Why are imm shifts where imm >= width type eliminated entirely?
...gt;
>> I'm just curious what the reasoning is for this? It isn't trying to set y
>> to anything at all.
>>
>
> The optimization is performed here:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/InstructionSimplify.cpp?revision=233938&view=markup#l1278
>
> What will happen is:
> %shr = lshr i16, %x, 129
> store i16 %shr, i16* @y
>
> will get transformed into:
> store i16 undef, i16* @y
>
> Then we will delete the store of undef using the following:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms...
2015 Apr 20
3
[LLVMdev] Why are imm shifts where imm >= width type eliminated entirely?
For example:
unsigned int x, y;
void foo()
{
y = x >> 129;
}
Where int is a 16bit type, the .ll is producing only 'ret void' at O3. At
O0 the .ll looks fine but then llc gets rid of it an simply returns.
I'm just curious what the reasoning is for this? It isn't trying to set y
to anything at all.
Thanks.
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