[PATCH] D112634: [InstCombine] canonicalize icmp with trunc op into mask and cmp, part 2

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In D112634#3965504 <https://reviews.llvm.org/D112634#3965504>, @MatzeB wrote:

> It seems this leads to slightly worse x86 codegen. Where we previously had:
>
>   %v1 = trunc i64 %v0 to i32
>   %cmp = icmp slt i32 %v1, 0
>
> resulting in:
>
>   testl   %eax, %eax
>   js      .LBB0_1
>
> instcombine now changes this to:
>
>   %v1 = and i64 %v0, 2147483648
>   %cmp = icmp eq i64 %v1, 0
>
> resulting in
>
>   testl   $-2147483648, %eax              # imm = 0x80000000
>   jne     .LBB0_2
>
> Though I guess we best fix this by adding more x86 patterns...

Yes, we can fix this up at some point in codegen/isel. For any mask+cmp of an i64/i32/i16 try to convert to a signbit test of a smaller power-of-2 type to avoid using a constant in the test instruction?


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  https://reviews.llvm.org/D112634/new/

https://reviews.llvm.org/D112634