[llvm] [SPIR-V] Implement selection for llvm.canonicalize(x) as fmul(x, 1.0) (PR #178439)

[Matt Arsenault via llvm-commits]

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@@ -3007,6 +3012,33 @@ SPIRVInstructionSelector::buildI32Constant(uint32_t Val, MachineInstr &I,
   return {NewReg, Result};
 }
 
+bool SPIRVInstructionSelector::selectFCanonicalize(Register ResVReg,
+                                                   const SPIRVType *ResType,
+                                                   MachineInstr &I) const {
+  // There is no native fcanonicalize instruction in SPIRV. We can lower it to:
+  // - fmin(x, x) or
+  // - fmul(x, 1.0)
+  //
+  // We use fmul(x, 1.0) here, because:
+  // - llvm-spirv translates fmin to a function call, whereas
+  //   fmul is translated to the LLVM fmul instruction.
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arsenm wrote:

I have been questioning whether we should keep maintaining this system of permitting canonicalize dropping. However, that still would not imply mandating fmul flush under a flushing environment. At minimum I think we need to stop allowing canonicalize dropping in codegen 

https://github.com/llvm/llvm-project/pull/178439