[llvm] ad91473 - [InstCombine] Improve eq/ne by parts to handle `ult/ugt` equality pattern.

[Noah Goldstein via llvm-commits]

Author: Noah Goldstein
Date: 2023-11-04T19:00:28-05:00
New Revision: ad9147399f196c1c9b6bec373a5ad3afadc51a4a

URL: https://github.com/llvm/llvm-project/commit/ad9147399f196c1c9b6bec373a5ad3afadc51a4a
DIFF: https://github.com/llvm/llvm-project/commit/ad9147399f196c1c9b6bec373a5ad3afadc51a4a.diff

LOG: [InstCombine] Improve eq/ne by parts to handle `ult/ugt` equality pattern.

(icmp eq/ne (lshr x, C), (lshr y, C) gets optimized to `(icmp
ult/uge (xor x, y), (1 << C)`. This can cause the current equal by
parts detection to miss the high-bits as it may get optimized to the
new pattern.

This commit adds support for detecting / combining the ult/ugt
pattern.

Closes #69884

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
    llvm/test/Transforms/InstCombine/eq-of-parts.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index 070d386b2f18d24..46af9bf5eed003a 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -1146,13 +1146,40 @@ Value *InstCombinerImpl::foldEqOfParts(ICmpInst *Cmp0, ICmpInst *Cmp1,
     return nullptr;
 
   CmpInst::Predicate Pred = IsAnd ? CmpInst::ICMP_EQ : CmpInst::ICMP_NE;
-  if (Cmp0->getPredicate() != Pred || Cmp1->getPredicate() != Pred)
-    return nullptr;
+  auto GetMatchPart = [&](ICmpInst *Cmp,
+                          unsigned OpNo) -> std::optional<IntPart> {
+    if (Pred == Cmp->getPredicate())
+      return matchIntPart(Cmp->getOperand(OpNo));
+
+    const APInt *C;
+    // (icmp eq (lshr x, C), (lshr y, C)) gets optimized to:
+    // (icmp ult (xor x, y), 1 << C) so also look for that.
+    if (Pred == CmpInst::ICMP_EQ && Cmp->getPredicate() == CmpInst::ICMP_ULT) {
+      if (!match(Cmp->getOperand(1), m_Power2(C)) ||
+          !match(Cmp->getOperand(0), m_Xor(m_Value(), m_Value())))
+        return std::nullopt;
+    }
+
+    // (icmp ne (lshr x, C), (lshr y, C)) gets optimized to:
+    // (icmp ugt (xor x, y), (1 << C) - 1) so also look for that.
+    else if (Pred == CmpInst::ICMP_NE &&
+             Cmp->getPredicate() == CmpInst::ICMP_UGT) {
+      if (!match(Cmp->getOperand(1), m_LowBitMask(C)) ||
+          !match(Cmp->getOperand(0), m_Xor(m_Value(), m_Value())))
+        return std::nullopt;
+    } else {
+      return std::nullopt;
+    }
+
+    unsigned From = Pred == CmpInst::ICMP_NE ? C->popcount() : C->countr_zero();
+    Instruction *I = cast<Instruction>(Cmp->getOperand(0));
+    return {{I->getOperand(OpNo), From, C->getBitWidth() - From}};
+  };
 
-  std::optional<IntPart> L0 = matchIntPart(Cmp0->getOperand(0));
-  std::optional<IntPart> R0 = matchIntPart(Cmp0->getOperand(1));
-  std::optional<IntPart> L1 = matchIntPart(Cmp1->getOperand(0));
-  std::optional<IntPart> R1 = matchIntPart(Cmp1->getOperand(1));
+  std::optional<IntPart> L0 = GetMatchPart(Cmp0, 0);
+  std::optional<IntPart> R0 = GetMatchPart(Cmp0, 1);
+  std::optional<IntPart> L1 = GetMatchPart(Cmp1, 0);
+  std::optional<IntPart> R1 = GetMatchPart(Cmp1, 1);
   if (!L0 || !R0 || !L1 || !R1)
     return nullptr;
 

diff  --git a/llvm/test/Transforms/InstCombine/eq-of-parts.ll b/llvm/test/Transforms/InstCombine/eq-of-parts.ll
index 5c220bde187d082..57b15ae3b96e66e 100644
--- a/llvm/test/Transforms/InstCombine/eq-of-parts.ll
+++ b/llvm/test/Transforms/InstCombine/eq-of-parts.ll
@@ -1336,12 +1336,7 @@ define i1 @ne_21_wrong_pred2(i32 %x, i32 %y) {
 
 define i1 @eq_optimized_highbits_cmp(i32 %x, i32 %y) {
 ; CHECK-LABEL: @eq_optimized_highbits_cmp(
-; CHECK-NEXT:    [[XOR:%.*]] = xor i32 [[Y:%.*]], [[X:%.*]]
-; CHECK-NEXT:    [[CMP_HI:%.*]] = icmp ult i32 [[XOR]], 33554432
-; CHECK-NEXT:    [[TX:%.*]] = trunc i32 [[X]] to i25
-; CHECK-NEXT:    [[TY:%.*]] = trunc i32 [[Y]] to i25
-; CHECK-NEXT:    [[CMP_LO:%.*]] = icmp eq i25 [[TX]], [[TY]]
-; CHECK-NEXT:    [[R:%.*]] = and i1 [[CMP_HI]], [[CMP_LO]]
+; CHECK-NEXT:    [[R:%.*]] = icmp eq i32 [[Y:%.*]], [[X:%.*]]
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %xor = xor i32 %y, %x
@@ -1393,12 +1388,7 @@ define i1 @eq_optimized_highbits_cmp_fail_not_pow2(i32 %x, i32 %y) {
 
 define i1 @ne_optimized_highbits_cmp(i32 %x, i32 %y) {
 ; CHECK-LABEL: @ne_optimized_highbits_cmp(
-; CHECK-NEXT:    [[XOR:%.*]] = xor i32 [[Y:%.*]], [[X:%.*]]
-; CHECK-NEXT:    [[CMP_HI:%.*]] = icmp ugt i32 [[XOR]], 16777215
-; CHECK-NEXT:    [[TX:%.*]] = trunc i32 [[X]] to i24
-; CHECK-NEXT:    [[TY:%.*]] = trunc i32 [[Y]] to i24
-; CHECK-NEXT:    [[CMP_LO:%.*]] = icmp ne i24 [[TX]], [[TY]]
-; CHECK-NEXT:    [[R:%.*]] = or i1 [[CMP_HI]], [[CMP_LO]]
+; CHECK-NEXT:    [[R:%.*]] = icmp ne i32 [[Y:%.*]], [[X:%.*]]
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %xor = xor i32 %y, %x