[clang] [llvm] [SPIRV] Add PreLegalizer pattern matching for `faceforward` (PR #139959)

[Farzon Lotfi via cfe-commits]

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@@ -58,3 +58,106 @@ void SPIRVCombinerHelper::applySPIRVDistance(MachineInstr &MI) const {
 
   MI.eraseFromParent();
 }
+
+/// This match is part of a combine that
+/// rewrites select(fcmp(dot(I, Ng), 0), N, -N) to faceforward(N, I, Ng)
+///   (vXf32 (g_select
+///             (g_fcmp
+///                (g_intrinsic dot(vXf32 I) (vXf32 Ng)
+///                 0)
+///             (vXf32 N)
+///             (vXf32 g_fneg (vXf32 N))))
+/// ->
+///   (vXf32 (g_intrinsic faceforward
+///             (vXf32 N) (vXf32 I) (vXf32 Ng)))
+///
+/// This only works for Vulkan targets.
+///
+bool SPIRVCombinerHelper::matchSelectToFaceForward(MachineInstr &MI) const {
+  if (!STI.isShader())
+    return false;
+
+  // Match overall select pattern.
+  Register CondReg, TrueReg, FalseReg;
+  if (!mi_match(MI.getOperand(0).getReg(), MRI,
+                m_GISelect(m_Reg(CondReg), m_Reg(TrueReg), m_Reg(FalseReg))))
+    return false;
+
+  // Match the FCMP condition.
+  Register DotReg, CondZeroReg;
+  CmpInst::Predicate Pred;
+  if (!mi_match(CondReg, MRI,
+                m_GFCmp(m_Pred(Pred), m_Reg(DotReg), m_Reg(CondZeroReg))) ||
+      Pred != CmpInst::FCMP_OLT)
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farzonl wrote:

should this be overly specialized to ordered and less than? What about these 16 other fcmp  condition codes? https://llvm.org/docs/LangRef.html#fcmp-instruction

https://github.com/llvm/llvm-project/pull/139959