[IndVars] Fix high-cost-expand check in LFTR #125828

artagnon · 2025-02-05T10:10:43Z

Call SCEVExpander::isHighCostExpansion on the expression that's actually expanded.

llvmbot · 2025-02-05T10:12:26Z

@llvm/pr-subscribers-llvm-transforms

Author: Ramkumar Ramachandra (artagnon)

Changes

Guard against high-cost expansions in genLoopLimit, by checking IVLimit against SCEVExpander::isHighCostExpansion.

Full diff: https://github.com/llvm/llvm-project/pull/125828.diff

4 Files Affected:

(modified) llvm/lib/Transforms/Scalar/IndVarSimplify.cpp (+9-3)
(modified) llvm/test/Transforms/IndVarSimplify/2011-11-01-lftrptr.ll (+3-6)
(modified) llvm/test/Transforms/IndVarSimplify/lftr-pr41998.ll (+5-7)
(modified) llvm/test/Transforms/IndVarSimplify/rewrite-loop-exit-values-phi.ll (+4-5)

diff --git a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
index 9619dfdbf41231..c3a9fa969eb3d3 100644
--- a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -912,7 +912,8 @@ static PHINode *FindLoopCounter(Loop *L, BasicBlock *ExitingBB,
 /// is taken ExitCount times.
 static Value *genLoopLimit(PHINode *IndVar, BasicBlock *ExitingBB,
                            const SCEV *ExitCount, bool UsePostInc, Loop *L,
-                           SCEVExpander &Rewriter, ScalarEvolution *SE) {
+                           SCEVExpander &Rewriter, ScalarEvolution *SE,
+                           const TargetTransformInfo *TTI) {
   assert(isLoopCounter(IndVar, L, SE));
   assert(ExitCount->getType()->isIntegerTy() && "exit count must be integer");
   const SCEVAddRecExpr *AR = cast<SCEVAddRecExpr>(SE->getSCEV(IndVar));
@@ -935,6 +936,9 @@ static Value *genLoopLimit(PHINode *IndVar, BasicBlock *ExitingBB,
   const SCEV *IVLimit = ARBase->evaluateAtIteration(ExitCount, *SE);
   assert(SE->isLoopInvariant(IVLimit, L) &&
          "Computed iteration count is not loop invariant!");
+  if (Rewriter.isHighCostExpansion(IVLimit, L, SCEVCheapExpansionBudget, TTI,
+                                   IndVar))
+    return nullptr;
   return Rewriter.expandCodeFor(IVLimit, ARBase->getType(),
                                 ExitingBB->getTerminator());
 }
@@ -995,8 +999,10 @@ linearFunctionTestReplace(Loop *L, BasicBlock *ExitingBB,
       BO->setHasNoSignedWrap(AR->hasNoSignedWrap());
   }
 
-  Value *ExitCnt = genLoopLimit(
-      IndVar, ExitingBB, ExitCount, UsePostInc, L, Rewriter, SE);
+  Value *ExitCnt = genLoopLimit(IndVar, ExitingBB, ExitCount, UsePostInc, L,
+                                Rewriter, SE, TTI);
+  if (!ExitCnt)
+    return false;
   assert(ExitCnt->getType()->isPointerTy() ==
              IndVar->getType()->isPointerTy() &&
          "genLoopLimit missed a cast");
diff --git a/llvm/test/Transforms/IndVarSimplify/2011-11-01-lftrptr.ll b/llvm/test/Transforms/IndVarSimplify/2011-11-01-lftrptr.ll
index cb0be523c70955..48e5c589797dc9 100644
--- a/llvm/test/Transforms/IndVarSimplify/2011-11-01-lftrptr.ll
+++ b/llvm/test/Transforms/IndVarSimplify/2011-11-01-lftrptr.ll
@@ -158,17 +158,14 @@ define i8 @testnullptrint(ptr %buf, ptr %end) nounwind {
 ; PTR64-NEXT:    [[GUARD:%.*]] = icmp ult i32 0, [[CNT]]
 ; PTR64-NEXT:    br i1 [[GUARD]], label [[PREHEADER:%.*]], label [[EXIT:%.*]]
 ; PTR64:       preheader:
-; PTR64-NEXT:    [[TMP1:%.*]] = add i32 [[EI]], -1
-; PTR64-NEXT:    [[TMP2:%.*]] = sub i32 [[TMP1]], [[BI]]
-; PTR64-NEXT:    [[TMP3:%.*]] = zext i32 [[TMP2]] to i64
-; PTR64-NEXT:    [[TMP4:%.*]] = add nuw nsw i64 [[TMP3]], 1
-; PTR64-NEXT:    [[SCEVGEP:%.*]] = getelementptr i8, ptr null, i64 [[TMP4]]
 ; PTR64-NEXT:    br label [[LOOP:%.*]]
 ; PTR64:       loop:
 ; PTR64-NEXT:    [[P_01_US_US:%.*]] = phi ptr [ null, [[PREHEADER]] ], [ [[GEP:%.*]], [[LOOP]] ]
+; PTR64-NEXT:    [[IV:%.*]] = phi i32 [ 0, [[PREHEADER]] ], [ [[IVNEXT:%.*]], [[LOOP]] ]
 ; PTR64-NEXT:    [[GEP]] = getelementptr inbounds i8, ptr [[P_01_US_US]], i64 1
 ; PTR64-NEXT:    [[SNEXT:%.*]] = load i8, ptr [[GEP]], align 1
-; PTR64-NEXT:    [[EXITCOND:%.*]] = icmp ne ptr [[GEP]], [[SCEVGEP]]
+; PTR64-NEXT:    [[IVNEXT]] = add nuw i32 [[IV]], 1
+; PTR64-NEXT:    [[EXITCOND:%.*]] = icmp ult i32 [[IVNEXT]], [[CNT]]
 ; PTR64-NEXT:    br i1 [[EXITCOND]], label [[LOOP]], label [[EXIT_LOOPEXIT:%.*]]
 ; PTR64:       exit.loopexit:
 ; PTR64-NEXT:    [[SNEXT_LCSSA:%.*]] = phi i8 [ [[SNEXT]], [[LOOP]] ]
diff --git a/llvm/test/Transforms/IndVarSimplify/lftr-pr41998.ll b/llvm/test/Transforms/IndVarSimplify/lftr-pr41998.ll
index b7f4756b2757fd..636ea9f53042e5 100644
--- a/llvm/test/Transforms/IndVarSimplify/lftr-pr41998.ll
+++ b/llvm/test/Transforms/IndVarSimplify/lftr-pr41998.ll
@@ -41,17 +41,15 @@ end:
 define void @test_ptr(i32 %start) {
 ; CHECK-LABEL: @test_ptr(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[TMP0:%.*]] = trunc i32 [[START:%.*]] to i3
-; CHECK-NEXT:    [[TMP1:%.*]] = sub i3 -1, [[TMP0]]
-; CHECK-NEXT:    [[TMP2:%.*]] = zext i3 [[TMP1]] to i64
-; CHECK-NEXT:    [[TMP3:%.*]] = add nuw nsw i64 [[TMP2]], 1
-; CHECK-NEXT:    [[UGLYGEP:%.*]] = getelementptr i8, ptr @data, i64 [[TMP3]]
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[P:%.*]] = phi ptr [ @data, [[ENTRY:%.*]] ], [ [[P_INC:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[I:%.*]] = phi i32 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[I_INC:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[P:%.*]] = phi ptr [ @data, [[ENTRY]] ], [ [[P_INC:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[I_INC]] = add nuw i32 [[I]], 1
 ; CHECK-NEXT:    [[P_INC]] = getelementptr inbounds i8, ptr [[P]], i64 1
 ; CHECK-NEXT:    store volatile i8 0, ptr [[P_INC]], align 1
-; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp eq ptr [[P_INC]], [[UGLYGEP]]
+; CHECK-NEXT:    [[AND:%.*]] = and i32 [[I_INC]], 7
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp eq i32 [[AND]], 0
 ; CHECK-NEXT:    br i1 [[EXITCOND]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
 ; CHECK-NEXT:    ret void
diff --git a/llvm/test/Transforms/IndVarSimplify/rewrite-loop-exit-values-phi.ll b/llvm/test/Transforms/IndVarSimplify/rewrite-loop-exit-values-phi.ll
index 37bc67c23adb75..4112b85e43e20b 100644
--- a/llvm/test/Transforms/IndVarSimplify/rewrite-loop-exit-values-phi.ll
+++ b/llvm/test/Transforms/IndVarSimplify/rewrite-loop-exit-values-phi.ll
@@ -14,20 +14,20 @@ define dso_local void @hoge() local_unnamed_addr {
 ; CHECK-LABEL: @hoge(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[N:%.*]] = sdiv exact i64 undef, 40
-; CHECK-NEXT:    [[TMP0:%.*]] = sub i64 undef, [[N]]
 ; CHECK-NEXT:    br label [[HEADER:%.*]]
 ; CHECK:       header:
-; CHECK-NEXT:    [[INDVARS_IV:%.*]] = phi i64 [ [[INDVARS_IV_NEXT:%.*]], [[LATCH:%.*]] ], [ [[TMP0]], [[ENTRY:%.*]] ]
-; CHECK-NEXT:    [[IDX:%.*]] = phi i64 [ [[IDX_NEXT:%.*]], [[LATCH]] ], [ undef, [[ENTRY]] ]
+; CHECK-NEXT:    [[IDX:%.*]] = phi i64 [ [[IDX_NEXT:%.*]], [[LATCH:%.*]] ], [ undef, [[ENTRY:%.*]] ]
 ; CHECK-NEXT:    [[COND:%.*]] = icmp sgt i64 [[N]], [[IDX]]
 ; CHECK-NEXT:    br i1 [[COND]], label [[END:%.*]], label [[INNER_PREHEADER:%.*]]
 ; CHECK:       inner.preheader:
 ; CHECK-NEXT:    br label [[INNER:%.*]]
 ; CHECK:       inner:
 ; CHECK-NEXT:    [[I:%.*]] = phi i64 [ [[I_NEXT:%.*]], [[INNER]] ], [ 0, [[INNER_PREHEADER]] ]
+; CHECK-NEXT:    [[J:%.*]] = phi i64 [ [[J_NEXT:%.*]], [[INNER]] ], [ [[N]], [[INNER_PREHEADER]] ]
 ; CHECK-NEXT:    [[I_NEXT]] = add nuw i64 [[I]], 1
+; CHECK-NEXT:    [[J_NEXT]] = add nsw i64 [[J]], 1
 ; CHECK-NEXT:    store i64 undef, ptr @ptr, align 8
-; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ne i64 [[I_NEXT]], [[INDVARS_IV]]
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp slt i64 [[J]], [[IDX]]
 ; CHECK-NEXT:    br i1 [[EXITCOND]], label [[INNER]], label [[INNER_EXIT:%.*]]
 ; CHECK:       inner_exit:
 ; CHECK-NEXT:    [[INDVAR:%.*]] = phi i64 [ [[I_NEXT]], [[INNER]] ]
@@ -35,7 +35,6 @@ define dso_local void @hoge() local_unnamed_addr {
 ; CHECK-NEXT:    br label [[LATCH]]
 ; CHECK:       latch:
 ; CHECK-NEXT:    [[IDX_NEXT]] = add nsw i64 [[IDX]], -1
-; CHECK-NEXT:    [[INDVARS_IV_NEXT]] = add i64 [[INDVARS_IV]], -1
 ; CHECK-NEXT:    br label [[HEADER]]
 ; CHECK:       end:
 ; CHECK-NEXT:    ret void

github-actions · 2025-02-05T10:18:14Z

✅ With the latest revision this PR passed the undef deprecator.

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

artagnon · 2025-02-06T12:07:40Z

Pending update of Transforms/PhaseOrdering/runtime-check-removal.ll.

artagnon · 2025-02-07T10:01:59Z

Rebased and ready to review.

nikic

Hm, it seems like costing IVLimit instead of ExitCount ends up having the opposite effect and we do more LFTR? Is IVLimit usually a simpler expression than ExitCount?

artagnon · 2025-02-10T15:52:48Z

Hm, it seems like costing IVLimit instead of ExitCount ends up having the opposite effect and we do more LFTR? Is IVLimit usually a simpler expression than ExitCount?

It's neither strictly more or strictly less complex. In some cases, we do more LFTR; in others, we do less LFTR. Considering that we're trying to phase out LFTR gradually, maybe I can add a FIXME high-cost-check with ExitCount as well, to avoid regressions?

artagnon · 2025-02-17T11:25:49Z

Gentle ping.

artagnon · 2025-02-26T13:33:23Z

Gentle ping.

artagnon · 2025-03-25T11:15:33Z

Gentle ping.

Guard against high-cost expansions in genLoopLimit, by checking IVLimit against SCEVExpander::isHighCostExpansion.

artagnon · 2025-06-20T12:49:14Z

Gentle ping. I think we ultimately want #126086, although we're still undecided on it, but I think this PR will give us many small improvements, and I think it is a step in the right direction.

artagnon requested review from nikic and dtcxzyw February 5, 2025 10:10

llvmbot added the llvm:transforms label Feb 5, 2025

nikic mentioned this pull request Feb 5, 2025

Task submission dtcxzyw/llvm-opt-benchmark#1312

Open

dtcxzyw mentioned this pull request Feb 5, 2025

pre-commit: PR125828 dtcxzyw/llvm-opt-benchmark#2087

Closed

artagnon mentioned this pull request Feb 5, 2025

SCEV: teach isImpliedViaOperations about samesign #124270

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nikic reviewed Feb 5, 2025

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llvm/lib/Transforms/Scalar/IndVarSimplify.cpp Outdated Show resolved Hide resolved

artagnon changed the title ~~IndVarSimplify: don't high-cost-expand in genLoopLimit~~ IndVarSimplify: fix high-cost-expand check Feb 5, 2025

dtcxzyw mentioned this pull request Feb 5, 2025

pre-commit: PR125828 dtcxzyw/llvm-opt-benchmark#2089

Closed

artagnon force-pushed the ivs-genlooplimit-cost branch from ea6d845 to 13f16d1 Compare February 7, 2025 10:01

nikic reviewed Feb 10, 2025

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artagnon force-pushed the ivs-genlooplimit-cost branch from 13f16d1 to c91471b Compare February 10, 2025 16:26

artagnon changed the title ~~IndVarSimplify: fix high-cost-expand check~~ IndVars: fix high-cost-expand check in LFTR Feb 10, 2025

artagnon changed the title ~~IndVars: fix high-cost-expand check in LFTR~~ [IndVars] Fix high-cost-expand check in LFTR Feb 26, 2025

artagnon requested a review from fhahn March 8, 2025 20:38

artagnon removed the request for review from fhahn June 20, 2025 12:03

IndVarSimplify: don't high-cost-expand in genLoopLimit

6d428b6

Guard against high-cost expansions in genLoopLimit, by checking IVLimit against SCEVExpander::isHighCostExpansion.

artagnon force-pushed the ivs-genlooplimit-cost branch from c91471b to 6d428b6 Compare June 20, 2025 12:46

artagnon requested review from preames and topperc June 20, 2025 12:46

zyw-bot mentioned this pull request Jun 20, 2025

pre-commit: PR125828 dtcxzyw/llvm-opt-benchmark#2475

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