[llvm-dev] How to prevent llvm's default optimization

[Ben Shi via llvm-dev]

Hi, James,


Thanks for your reply. 


I do not think it is always true, that "mul then add" is faster than "add then mul".


For example,


A small immediate can be directly encoded in the instruction, but it becomes a larger one after a multiplication, which has to be loaded from the constant pool (extra memory access).


So I wonder, is it possile to prevent it, via changes to the config of the base class TargetLowering，than writing special custom C++ code.




Ben

















At 2020-06-30 07:46:41, "James Courtier-Dutton" <james.dutton at gmail.com> wrote:
>Hi Ben,
>
>Why do you want to stop it?
>"mul then add" is faster than "add then mul".
>The result is the same in both cases.
>
>On Mon, 29 Jun 2020 at 22:11, Ben Shi via llvm-dev
><llvm-dev at lists.llvm.org> wrote:
>>
>> Hello,
>> I have an instruction pattern like
>>   %2 = add i32 %0, 25
>>   %3 = mul i32 %2, 525
>>
>> and llvm will optimize it to
>> %2 = mul i32 %0, 525
>> %3 = add i32 %2, 12525
>>
>> how to prevent it?
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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