[llvm] 8fab9f8 - [IndVars] Sharpen context in eliminateIVComparison

[Max Kazantsev via llvm-commits]

Author: Max Kazantsev
Date: 2021-03-22T11:55:57+07:00
New Revision: 8fab9f824fcaa38edebbd0f6b86f8612a4212cdd

URL: https://github.com/llvm/llvm-project/commit/8fab9f824fcaa38edebbd0f6b86f8612a4212cdd
DIFF: https://github.com/llvm/llvm-project/commit/8fab9f824fcaa38edebbd0f6b86f8612a4212cdd.diff

LOG: [IndVars] Sharpen context in eliminateIVComparison

When eliminating comparisons, we can use common dominator of
all its users as context. This gives better results when ICMP is not
computed right before the branch that uses it.

Differential Revision: https://reviews.llvm.org/D98924
Reviewed By: lebedev.ri

Added: 
    

Modified: 
    llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
    llvm/test/Transforms/IndVarSimplify/eliminate-comparison.ll

Removed: 
    


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diff  --git a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
index f0e446684801..d4f325cd4682 100644
--- a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -281,8 +281,11 @@ void SimplifyIndvar::eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand) {
 
   // If the condition is always true or always false in the given context,
   // replace it with a constant value.
-  // TODO: We can sharpen the context to common dominator of all ICmp's users.
-  if (auto Ev = SE->evaluatePredicateAt(Pred, S, X, ICmp)) {
+  SmallVector<Instruction *, 4> Users;
+  for (auto *U : ICmp->users())
+    Users.push_back(cast<Instruction>(U));
+  const Instruction *CtxI = findCommonDominator(Users, *DT);
+  if (auto Ev = SE->evaluatePredicateAt(Pred, S, X, CtxI)) {
     ICmp->replaceAllUsesWith(ConstantInt::getBool(ICmp->getContext(), *Ev));
     DeadInsts.emplace_back(ICmp);
     LLVM_DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');

diff  --git a/llvm/test/Transforms/IndVarSimplify/eliminate-comparison.ll b/llvm/test/Transforms/IndVarSimplify/eliminate-comparison.ll
index 48a51d723d24..ee864b77ba2e 100644
--- a/llvm/test/Transforms/IndVarSimplify/eliminate-comparison.ll
+++ b/llvm/test/Transforms/IndVarSimplify/eliminate-comparison.ll
@@ -1101,13 +1101,11 @@ define i32 @func_28(i32 %start) {
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[BACKEDGE:%.*]] ]
 ; CHECK-NEXT:    [[C1:%.*]] = icmp ne i32 [[IV]], 0
-; CHECK-NEXT:    [[C2:%.*]] = icmp ne i32 [[IV]], 0
-; CHECK-NEXT:    [[C3:%.*]] = icmp ne i32 [[IV]], 0
 ; CHECK-NEXT:    br i1 [[C1]], label [[CHECKED_1:%.*]], label [[FAIL:%.*]]
 ; CHECK:       checked.1:
-; CHECK-NEXT:    br i1 [[C2]], label [[CHECKED_2:%.*]], label [[FAIL]]
+; CHECK-NEXT:    br i1 true, label [[CHECKED_2:%.*]], label [[FAIL]]
 ; CHECK:       checked.2:
-; CHECK-NEXT:    br i1 [[C3]], label [[BACKEDGE]], label [[FAIL]]
+; CHECK-NEXT:    br i1 true, label [[BACKEDGE]], label [[FAIL]]
 ; CHECK:       backedge:
 ; CHECK-NEXT:    [[IV_NEXT]] = add i32 [[IV]], 758394
 ; CHECK-NEXT:    [[LOOP_COND:%.*]] = call i1 @cond_func()