[llvm] r209162 - [ConstantHoisting][X86] Change the cost model to never hoist constants for types larger than i128.

[Quentin Colombet]

Hi Juergen,

Could you file a PR for the actual issue?

Thanks,
-Quentin

On May 19, 2014, at 2:00 PM, Juergen Ributzka <juergen at apple.com> wrote:

> Author: ributzka
> Date: Mon May 19 16:00:53 2014
> New Revision: 209162
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=209162&view=rev
> Log:
> [ConstantHoisting][X86] Change the cost model to never hoist constants for types larger than i128.
> 
> Currently the X86 backend doesn't support types larger than i128 very well. For
> example an i192 multiply will assert in codegen when the 2nd argument is a constant and the constant got hoisted.
> 
> This fix changes the cost model to never hoist constants for types larger than
> i128. Once the codegen issues have been resolved, the cost model can be updated
> to allow also larger types.
> 
> This is related to <rdar://problem/16954938>
> 
> Modified:
>    llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp
>    llvm/trunk/test/Transforms/ConstantHoisting/X86/large-immediate.ll
> 
> Modified: llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp?rev=209162&r1=209161&r2=209162&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp (original)
> +++ llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp Mon May 19 16:00:53 2014
> @@ -815,6 +815,13 @@ unsigned X86TTI::getIntImmCost(const API
>   if (BitSize == 0)
>     return ~0U;
> 
> +  // Never hoist constants larger than 128bit, because this might lead to
> +  // incorrect code generation or assertions in codegen.
> +  // Fixme: Create a cost model for types larger than i128 once the codegen
> +  // issues have been fixed.
> +  if (BitSize > 128)
> +    return TCC_Free;
> +
>   if (Imm == 0)
>     return TCC_Free;
> 
> @@ -830,8 +837,10 @@ unsigned X86TTI::getIntImmCost(unsigned
>   assert(Ty->isIntegerTy());
> 
>   unsigned BitSize = Ty->getPrimitiveSizeInBits();
> +  // There is no cost model for constants with a bit size of 0. Return TCC_Free
> +  // here, so that constant hoisting will ignore this constant.
>   if (BitSize == 0)
> -    return ~0U;
> +    return TCC_Free;
> 
>   unsigned ImmIdx = ~0U;
>   switch (Opcode) {
> @@ -892,8 +901,10 @@ unsigned X86TTI::getIntImmCost(Intrinsic
>   assert(Ty->isIntegerTy());
> 
>   unsigned BitSize = Ty->getPrimitiveSizeInBits();
> +  // There is no cost model for constants with a bit size of 0. Return TCC_Free
> +  // here, so that constant hoisting will ignore this constant.
>   if (BitSize == 0)
> -    return ~0U;
> +    return TCC_Free;
> 
>   switch (IID) {
>   default: return TCC_Free;
> 
> Modified: llvm/trunk/test/Transforms/ConstantHoisting/X86/large-immediate.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/ConstantHoisting/X86/large-immediate.ll?rev=209162&r1=209161&r2=209162&view=diff
> ==============================================================================
> --- llvm/trunk/test/Transforms/ConstantHoisting/X86/large-immediate.ll (original)
> +++ llvm/trunk/test/Transforms/ConstantHoisting/X86/large-immediate.ll Mon May 19 16:00:53 2014
> @@ -16,3 +16,12 @@ define i512 @test2(i512 %a) nounwind {
>   %2 = ashr i512 %1, 504
>   ret i512 %2
> }
> +
> +; Check that we don't hoist constants with a type larger than i128.
> +define i196 @test3(i196 %a) nounwind {
> +; CHECK-LABEL: test3
> +; CHECK-NOT: %const = bitcast i196 2 to i196
> +  %1 = mul i196 %a, 2
> +  %2 = mul i196 %1, 2
> +  ret i196 %2
> +}
> 
> 
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