[llvm] [X86][GlobalISel] Add instruction selection for G_SELECT (PR #70753)

[Evgenii Kudriashov via llvm-commits]

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@@ -1789,6 +1793,47 @@ bool X86InstructionSelector::selectMulDivRem(MachineInstr &I,
   return true;
 }
 
+bool X86InstructionSelector::selectSelect(MachineInstr &I,
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e-kud wrote:

Yes. There are only patterns for `X86ISD::CMOV` so we need to copy-paste patterns for `ISD::SELECT`.
There is another way: we can legalize `G_SELECT` into `G_CMOV` but then we'll meet a problem that there is no register bank for `EFLAGS`. So we can't match instructions as we can't match with EFLAGS. It seems that most of the opcodes here are related to `EFLAGS`. This fact I've revealed only now. For sure it should be fixed so that EFLAGS matching works and we can reuse tblgen.

https://github.com/llvm/llvm-project/pull/70753