[llvm] 1a37147 - [SCEV] Match both (-1)b + a and a + (-1)b as a - b (#84247)

[via llvm-commits]

Author: Philip Reames
Date: 2024-03-06T15:57:34-08:00
New Revision: 1a37147af5a91635e3982a9cbbdfe1d7492af84c

URL: https://github.com/llvm/llvm-project/commit/1a37147af5a91635e3982a9cbbdfe1d7492af84c
DIFF: https://github.com/llvm/llvm-project/commit/1a37147af5a91635e3982a9cbbdfe1d7492af84c.diff

LOG: [SCEV] Match both (-1)b + a and a + (-1)b as a - b (#84247)

In our analysis of guarding conditions, we were converting a-b == 0 into
a == b alternate form, but we were only checking for one of the two
forms for the sub. There's no requirement that the multiply only be on
the LHS of the add.

Added: 
    

Modified: 
    llvm/lib/Analysis/ScalarEvolution.cpp
    llvm/test/Analysis/ScalarEvolution/trip-count.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 15c2965aede1a0..acc0aa23107bb5 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -10577,6 +10577,25 @@ static bool HasSameValue(const SCEV *A, const SCEV *B) {
   return false;
 }
 
+static bool MatchBinarySub(const SCEV *S, const SCEV *&LHS, const SCEV *&RHS) {
+  const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S);
+  if (!Add || Add->getNumOperands() != 2)
+    return false;
+  if (auto *ME = dyn_cast<SCEVMulExpr>(Add->getOperand(0));
+      ME && ME->getNumOperands() == 2 && ME->getOperand(0)->isAllOnesValue()) {
+    LHS = Add->getOperand(1);
+    RHS = ME->getOperand(1);
+    return true;
+  }
+  if (auto *ME = dyn_cast<SCEVMulExpr>(Add->getOperand(1));
+      ME && ME->getNumOperands() == 2 && ME->getOperand(0)->isAllOnesValue()) {
+    LHS = Add->getOperand(0);
+    RHS = ME->getOperand(1);
+    return true;
+  }
+  return false;
+}
+
 bool ScalarEvolution::SimplifyICmpOperands(ICmpInst::Predicate &Pred,
                                            const SCEV *&LHS, const SCEV *&RHS,
                                            unsigned Depth) {
@@ -10652,19 +10671,10 @@ bool ScalarEvolution::SimplifyICmpOperands(ICmpInst::Predicate &Pred,
       case ICmpInst::ICMP_EQ:
       case ICmpInst::ICMP_NE:
         // Fold ((-1) * %a) + %b == 0 (equivalent to %b-%a == 0) into %a == %b.
-        if (!RA)
-          if (const SCEVAddExpr *AE = dyn_cast<SCEVAddExpr>(LHS))
-            if (const SCEVMulExpr *ME =
-                    dyn_cast<SCEVMulExpr>(AE->getOperand(0)))
-              if (AE->getNumOperands() == 2 && ME->getNumOperands() == 2 &&
-                  ME->getOperand(0)->isAllOnesValue()) {
-                RHS = AE->getOperand(1);
-                LHS = ME->getOperand(1);
-                Changed = true;
-              }
+        if (RA.isZero() && MatchBinarySub(LHS, LHS, RHS))
+          Changed = true;
         break;
 
-
         // The "Should have been caught earlier!" messages refer to the fact
         // that the ExactCR.isFullSet() or ExactCR.isEmptySet() check above
         // should have fired on the corresponding cases, and canonicalized the

diff  --git a/llvm/test/Analysis/ScalarEvolution/trip-count.ll b/llvm/test/Analysis/ScalarEvolution/trip-count.ll
index cbe07effdeb265..8fc5b9b4096127 100644
--- a/llvm/test/Analysis/ScalarEvolution/trip-count.ll
+++ b/llvm/test/Analysis/ScalarEvolution/trip-count.ll
@@ -145,7 +145,7 @@ define void @dual_sext_ne_with_slt_guard(i8 %s, i8 %n) {
 ; CHECK-LABEL: 'dual_sext_ne_with_slt_guard'
 ; CHECK-NEXT:  Determining loop execution counts for: @dual_sext_ne_with_slt_guard
 ; CHECK-NEXT:  Loop %for.body: backedge-taken count is (-1 + (sext i8 %n to i64) + (-1 * (sext i8 %s to i64))<nsw>)
-; CHECK-NEXT:  Loop %for.body: constant max backedge-taken count is i64 -1
+; CHECK-NEXT:  Loop %for.body: constant max backedge-taken count is i64 -2
 ; CHECK-NEXT:  Loop %for.body: symbolic max backedge-taken count is (-1 + (sext i8 %n to i64) + (-1 * (sext i8 %s to i64))<nsw>)
 ; CHECK-NEXT:  Loop %for.body: Trip multiple is 1
 ;