[PATCH] D31398: [X86][X86 intrinsics]Folding cmp(sub(a, b), 0) into cmp(a, b) optimization

[Sanjay Patel via Phabricator via llvm-commits]

spatel added inline comments.


================
Comment at: Transforms/InstCombine/InstCombineCalls.cpp:2348-2366
+    for (int i = 0; i < 2; i++) {
+      // Folding cmp(sub(a,b),0) into cmp(a,b)
+      if (auto  *I = dyn_cast<Instruction>(II->getArgOperand(i))) {
+        if (I->getOpcode() == Instruction::FSub && I->hasOneUse()) {
+          // This fold requires only the NINF(not +/- inf) since inf minus
+          // inf is nan.
+          // NSZ(No Signed Zeros) is not needed because zeros of any sign are
----------------
There are many ways to deal with commuted patterns, and a 2-loop is my least favorite. Would you consider using std::swap and matchers instead? Something like:
      Value *Arg0 = II->getArgOperand(0);
      Value *Arg1 = II->getArgOperand(1);
      bool Arg0IsZero = match(Arg0, m_Zero());
      if (Arg0IsZero)
        std::swap(Arg0, Arg1);
      Value *A, *B;
      if ((match(Arg0, m_OneUse(m_FSub(m_Value(A), m_Value(B)))) &&
           match(Arg1, m_Zero()) &&
           cast<Instruction>(Arg0)->getFastMathFlags().noInfs())) {
        if (Arg0IsZero)
          std::swap(A, B);
        II->setArgOperand(0, A);
        II->setArgOperand(1, B);
        return II;
      }



https://reviews.llvm.org/D31398