[llvm] GlobalISel needs fdiv 1 / sqrt(x) to rsq combine (PR #78673)

[Pierre van Houtryve via llvm-commits]

================
@@ -334,6 +337,59 @@ bool AMDGPUPostLegalizerCombinerImpl::matchRcpSqrtToRsq(
   return false;
 }
 
+bool AMDGPUPostLegalizerCombinerImpl::matchFDivSqrt(
+    MachineInstr &MI,
+    std::function<void(MachineIRBuilder &)> &MatchInfo) const {
+
+  // TODO: Can I match fdiv 1.0 / sqrt(x) from here?
+  // My apologies, this code is still a mess. Trying to figure out
+  // what value MI should hold when getting to this point
+
+  auto getSqrtSrc = [=](const MachineInstr &MI) -> MachineInstr * {
+    if (!MI.getFlag(MachineInstr::FmContract))
+      return nullptr;
+    MachineInstr *SqrtSrcMI = nullptr;
+    auto Match =
+        mi_match(MI.getOperand(0).getReg(), MRI, m_GFSqrt(m_MInstr(SqrtSrcMI)));
+    (void)Match;
+    return SqrtSrcMI;
+  };
+
+  // Do I need to match write a matcher for  %one:_(s16) = G_FCONSTANT half 1.0
+  // ??
+
+  auto getFdivSrc = [=](const MachineInstr &MI) -> MachineInstr * {
+    if (!MI.getFlag(MachineInstr::FmContract))
+      return nullptr;
+
+    MachineInstr *FDivSrcMI = nullptr;
+    Register One;
+    auto Match = mi_match(MI.getOperand(0).getReg(), MRI,
+                          m_GFDiv(m_Reg(One), m_MInstr(FDivSrcMI)));
+    // Not sure how to check for FDiv operancd has a 1.0 value ?
----------------
Pierre-vh wrote:

https://llvm.org/doxygen/classllvm_1_1ConstantFP.html#a69943c1bc81f3680f6f696b6565853e5

https://github.com/llvm/llvm-project/pull/78673