[clang] [llvm] [HLSL] Implement the `fwidth` intrinsic for DXIL and SPIR-V target (PR #161378)

[Farzon Lotfi via llvm-commits]

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@@ -3584,10 +3583,13 @@ bool SPIRVInstructionSelector::selectIntrinsic(Register ResVReg,
     return selectExtInst(ResVReg, ResType, I, GL::UnpackHalf2x16);
   }
   case Intrinsic::spv_ddx_coarse: {
-    return selectDpdCoarse(ResVReg, ResType, I, SPIRV::OpDPdxCoarse);
+    return selectDerivativeInst(ResVReg, ResType, I, SPIRV::OpDPdxCoarse);
   }
   case Intrinsic::spv_ddy_coarse: {
-    return selectDpdCoarse(ResVReg, ResType, I, SPIRV::OpDPdyCoarse);
+    return selectDerivativeInst(ResVReg, ResType, I, SPIRV::OpDPdyCoarse);
+  }
+  case Intrinsic::spv_fwidth: {
+    return selectDerivativeInst(ResVReg, ResType, I, SPIRV::OpFwidth);
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farzonl wrote:

This file is not very consistent but some of these single return case statements don't have curly braces `{}`. This might be an ambigous part of the style guide: https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements. I do beleive this counts as a single statement body so if possible drop the curly braces.

https://github.com/llvm/llvm-project/pull/161378