[PATCH] D127871: [RISCV] Optimize 2x SELECT for floating-point types

Thu Jul 7 22:06:46 PDT 2022

craig.topper added a comment.

In D127871#3637829 <https://reviews.llvm.org/D127871#3637829>, @liaolucy wrote:

> In D127871#3637493 <https://reviews.llvm.org/D127871#3637493>, @craig.topper wrote:
>
>> I think the expanded branches are emitted in the wrong order. The condition for the last Select_FPR32_Using_CC_GPR needs to be checked first.
>
> Thanks, Craig. I dump all select, The middle three pairs of instructions can be optimized
>
>     %25:fpr32 = Select_FPR32_Using_CC_GPR killed %21:gpr, %24:gpr, 1, %16:fpr32, %23:fpr32
>     %26:fpr32 = Select_FPR32_Using_CC_GPR killed %20:gpr, %24:gpr, 1, %16:fpr32, %23:fpr32
>     %28:fpr32 = Select_FPR32_Using_CC_GPR killed %8:gpr, %27:gpr, 4, %4:fpr32, killed %26:fpr32
>   
>   optimize1 
>    1.1 %29:fpr32 = Select_FPR32_Using_CC_GPR killed %7:gpr, %27:gpr, 4, %3:fpr32, killed %25:fpr3
>    2.1 %30:fpr32 = Select_FPR32_Using_CC_GPR killed %18:gpr, %24:gpr, 1, %16:fpr32, killed %29:fpr32
>   optimize2
>    1.2 %31:fpr32 = Select_FPR32_Using_CC_GPR killed %17:gpr, %24:gpr, 1, %16:fpr32, killed %28:fpr32
>    2.2 %32:fpr32 = Select_FPR32_Using_CC_GPR killed %14:gpr, %24:gpr, 1, %19:fpr32, killed %31:fpr32
>   optimize3
>    1.3 %33:fpr32 = Select_FPR32_Using_CC_GPR killed %13:gpr, %24:gpr, 1, %19:fpr32, killed %30:fpr32
>    2.3 %34:fpr32 = Select_FPR32_Using_CC_GPR killed %10:gpr, %24:gpr, 0, %1:fpr32, killed %33:fpr32
>   
>     %35:fpr32 = Select_FPR32_Using_CC_GPR killed %9:gpr, %24:gpr, 0, %2:fpr32, killed %32:fpr32
>
> I think  we  should check select1-rs2 == select2-rs2, then 1.1 and 2.1 can not be optimized. I will update patch later

I dont' think checking select1-rs2 == select2-rs2 solves the problem. When the condition of the second select is true, the true value of that select has priority. That means when the select2 condition is true, the select1 condition matter. The patch is trying to prioritize one of the compares to skip the other the other. That means you need to check the select2 condition first to maintain the priority.

Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D127871/new/

https://reviews.llvm.org/D127871