[EarlyCSE] Improve EarlyCSE of some absolute value cases.

Change matchSelectPattern to return X and -X for ABS/NABS in a well defined order. Adjust EarlyCSE to account for this. Ensure the SPF result is some kind of min/max and not abs/nabs in one place in InstCombine that made me nervous.

Prevously we returned the two operands of the compare part of the abs pattern. The RHS is always going to be a 0i, 1 or -1 constant. This isn't a very meaningful thing to return for any one. There's also some freedom in the abs pattern as to what happens when the value is equal to 0. This freedom led to early cse failing to match when different constants were used in otherwise equivalent operations. By returning the input and its negation in a defined order we can ensure an exact match. This also makes sure both patterns use the exact same subtract instruction for the negation. I believe CSE should evebntually make this happen and properly merge the nsw/nuw flags. But I'm not familiar with CSE and what order it does things in so it seemed like it might be good to really enforce that they were the same.

Differential Revision: https://reviews.llvm.org/D47037

llvm-svn: 332865

1 files changed, 12 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
index d19ceab3889..a65d1556704 100644
--- a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
+++ b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -155,12 +155,15 @@ unsigned DenseMapInfo<SimpleValue>::getHashValue(SimpleValue Val) {
   SelectPatternFlavor SPF = matchSelectPattern(Inst, A, B).Flavor;
   // TODO: We should also detect FP min/max.
   if (SPF == SPF_SMIN || SPF == SPF_SMAX ||
-      SPF == SPF_UMIN || SPF == SPF_UMAX ||
-      SPF == SPF_ABS || SPF == SPF_NABS) {
+      SPF == SPF_UMIN || SPF == SPF_UMAX) {
     if (A > B)
       std::swap(A, B);
     return hash_combine(Inst->getOpcode(), SPF, A, B);
   }
+  if (SPF == SPF_ABS || SPF == SPF_NABS) {
+    // ABS/NABS always puts the input in A and its negation in B.
+    return hash_combine(Inst->getOpcode(), SPF, A, B);
+  }
 
   if (CastInst *CI = dyn_cast<CastInst>(Inst))
     return hash_combine(CI->getOpcode(), CI->getType(), CI->getOperand(0));
@@ -230,8 +233,13 @@ bool DenseMapInfo<SimpleValue>::isEqual(SimpleValue LHS, SimpleValue RHS) {
       LSPF == SPF_ABS || LSPF == SPF_NABS) {
     Value *RHSA, *RHSB;
     SelectPatternFlavor RSPF = matchSelectPattern(RHSI, RHSA, RHSB).Flavor;
-    return (LSPF == RSPF && ((LHSA == RHSA && LHSB == RHSB) ||
-                             (LHSA == RHSB && LHSB == RHSA)));
+    if (LSPF == RSPF) {
+      // Abs results are placed in a defined order by matchSelectPattern.
+      if (LSPF == SPF_ABS || LSPF == SPF_NABS)
+        return LHSA == RHSA && LHSB == RHSB;
+      return ((LHSA == RHSA && LHSB == RHSB) ||
+              (LHSA == RHSB && LHSB == RHSA));
+    }
   }
 
   return false;