[llvm] [LV] Don't vectorize if trip count expansion may introduce UB. (PR #92177)
Florian Hahn via llvm-commits
llvm-commits at lists.llvm.org
Tue May 14 14:04:47 PDT 2024
https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/92177
Introduce a utility to check if a SCEV expansion may introduce UB (couldn't find a similar utility after a quick glance) and use to the avoid vectorizing when expanding the trip count introduces UB.
Fixes https://github.com/llvm/llvm-project/issues/89958.
>From 49c699d0a832a12e9d93fbb581ac5dbda5eb6882 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Tue, 14 May 2024 20:36:16 +0100
Subject: [PATCH] [LV] Don't vectorize if trip count expansion may introduce
UB.
Introduce a utility to check if a SCEV expansion may introduce UB
(couldn't find a similar utility after a quick glance) and use to the
avoid vectorizing when expanding the trip count introduces UB.
Fixes https://github.com/llvm/llvm-project/issues/89958.
---
.../Utils/ScalarEvolutionExpander.h | 4 +
.../Utils/ScalarEvolutionExpander.cpp | 5 +
.../Vectorize/LoopVectorizationLegality.cpp | 26 +++++
.../trip-count-expansion-may-introduce-ub.ll | 100 ++----------------
4 files changed, 41 insertions(+), 94 deletions(-)
diff --git a/llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h b/llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h
index 62c1e15a9a60e..270286afebd2a 100644
--- a/llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h
+++ b/llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h
@@ -418,6 +418,10 @@ class SCEVExpander : public SCEVVisitor<SCEVExpander, Value *> {
BasicBlock::iterator findInsertPointAfter(Instruction *I,
Instruction *MustDominate) const;
+ /// Returns true if expanding \p Expr may introduce UB (e.g. because the
+ /// expression contains an UDiv expression).
+ static bool expansionMayIntroduceUB(const SCEV *Expr);
+
private:
LLVMContext &getContext() const { return SE.getContext(); }
diff --git a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
index 0feea0a4233cd..0a5ec8baccbee 100644
--- a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
@@ -161,6 +161,11 @@ SCEVExpander::findInsertPointAfter(Instruction *I,
return IP;
}
+bool SCEVExpander::expansionMayIntroduceUB(const SCEV *Expr) {
+ return SCEVExprContains(Expr,
+ [](const SCEV *Op) { return isa<SCEVUDivExpr>(Op); });
+}
+
BasicBlock::iterator
SCEVExpander::GetOptimalInsertionPointForCastOf(Value *V) const {
// Cast the argument at the beginning of the entry block, after
diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
index 9de49d1bcfeac..e1b1ad083f184 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
@@ -24,6 +24,7 @@
#include "llvm/Analysis/VectorUtils.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/PatternMatch.h"
+#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
#include "llvm/Transforms/Utils/SizeOpts.h"
#include "llvm/Transforms/Vectorize/LoopVectorize.h"
@@ -1506,6 +1507,31 @@ bool LoopVectorizationLegality::canVectorize(bool UseVPlanNativePath) {
return false;
}
+ SmallVector<BasicBlock *> Exiting;
+ TheLoop->getExitingBlocks(Exiting);
+ // Check if eexit count for any exit that may execute unconditionally may in
+ // introduce UB. Note that we can skip checks in the header or if there's a
+ // single exit, as in those cases we know that the exit count will be
+ // evaluated in each loop iteration. There are other cases where the exiting
+ // block executes on each loop iteration, but we don't have a cheap way to
+ // check at the moment.
+ ScalarEvolution &SE = *PSE.getSE();
+ if (Exiting.size() > 1 && any_of(Exiting, [this, &SE](BasicBlock *E) {
+ if (TheLoop->getHeader() == E)
+ return false;
+ const SCEV *EC = SE.getExitCount(TheLoop, E);
+ return !isa<SCEVCouldNotCompute>(EC) &&
+ SCEVExpander::expansionMayIntroduceUB(EC);
+ })) {
+ LLVM_DEBUG(
+ dbgs()
+ << "LV: Expanding the trip count expression may introduce UB.\n");
+ if (DoExtraAnalysis)
+ Result = false;
+ else
+ return false;
+ }
+
LLVM_DEBUG(dbgs() << "LV: We can vectorize this loop"
<< (LAI->getRuntimePointerChecking()->Need
? " (with a runtime bound check)"
diff --git a/llvm/test/Transforms/LoopVectorize/trip-count-expansion-may-introduce-ub.ll b/llvm/test/Transforms/LoopVectorize/trip-count-expansion-may-introduce-ub.ll
index 85fad6fb36328..168c0f88e4fc0 100644
--- a/llvm/test/Transforms/LoopVectorize/trip-count-expansion-may-introduce-ub.ll
+++ b/llvm/test/Transforms/LoopVectorize/trip-count-expansion-may-introduce-ub.ll
@@ -74,66 +74,9 @@ define i64 @multi_exit_2_exit_count_with_udiv_in_block_executed_unconditionally(
; CHECK-LABEL: define i64 @multi_exit_2_exit_count_with_udiv_in_block_executed_unconditionally(
; CHECK-SAME: ptr [[A:%.*]], i64 [[N:%.*]]) {
; CHECK-NEXT: entry:
-; CHECK-NEXT: [[SMAX:%.*]] = call i64 @llvm.smax.i64(i64 [[N]], i64 0)
-; CHECK-NEXT: [[TMP0:%.*]] = udiv i64 42, [[N]]
-; CHECK-NEXT: [[UMIN:%.*]] = call i64 @llvm.umin.i64(i64 [[SMAX]], i64 [[TMP0]])
-; CHECK-NEXT: [[TMP1:%.*]] = add nuw nsw i64 [[UMIN]], 1
-; CHECK-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ule i64 [[TMP1]], 4
-; CHECK-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
-; CHECK: vector.ph:
-; CHECK-NEXT: [[N_MOD_VF:%.*]] = urem i64 [[TMP1]], 4
-; CHECK-NEXT: [[TMP2:%.*]] = icmp eq i64 [[N_MOD_VF]], 0
-; CHECK-NEXT: [[TMP3:%.*]] = select i1 [[TMP2]], i64 4, i64 [[N_MOD_VF]]
-; CHECK-NEXT: [[N_VEC:%.*]] = sub i64 [[TMP1]], [[TMP3]]
-; CHECK-NEXT: br label [[VECTOR_BODY:%.*]]
-; CHECK: vector.body:
-; CHECK-NEXT: [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[PRED_STORE_CONTINUE6:%.*]] ]
-; CHECK-NEXT: [[TMP4:%.*]] = add i64 [[INDEX]], 0
-; CHECK-NEXT: [[TMP5:%.*]] = getelementptr inbounds i32, ptr [[A]], i64 [[TMP4]]
-; CHECK-NEXT: [[TMP6:%.*]] = getelementptr inbounds i32, ptr [[TMP5]], i32 0
-; CHECK-NEXT: [[WIDE_LOAD:%.*]] = load <4 x i32>, ptr [[TMP6]], align 4
-; CHECK-NEXT: [[TMP7:%.*]] = icmp eq <4 x i32> [[WIDE_LOAD]], <i32 10, i32 10, i32 10, i32 10>
-; CHECK-NEXT: [[TMP8:%.*]] = extractelement <4 x i1> [[TMP7]], i32 0
-; CHECK-NEXT: br i1 [[TMP8]], label [[PRED_STORE_IF:%.*]], label [[PRED_STORE_CONTINUE:%.*]]
-; CHECK: pred.store.if:
-; CHECK-NEXT: [[TMP9:%.*]] = getelementptr inbounds i32, ptr [[A]], i64 [[TMP4]]
-; CHECK-NEXT: store i32 1, ptr [[TMP9]], align 4
-; CHECK-NEXT: br label [[PRED_STORE_CONTINUE]]
-; CHECK: pred.store.continue:
-; CHECK-NEXT: [[TMP10:%.*]] = extractelement <4 x i1> [[TMP7]], i32 1
-; CHECK-NEXT: br i1 [[TMP10]], label [[PRED_STORE_IF1:%.*]], label [[PRED_STORE_CONTINUE2:%.*]]
-; CHECK: pred.store.if1:
-; CHECK-NEXT: [[TMP11:%.*]] = add i64 [[INDEX]], 1
-; CHECK-NEXT: [[TMP12:%.*]] = getelementptr inbounds i32, ptr [[A]], i64 [[TMP11]]
-; CHECK-NEXT: store i32 1, ptr [[TMP12]], align 4
-; CHECK-NEXT: br label [[PRED_STORE_CONTINUE2]]
-; CHECK: pred.store.continue2:
-; CHECK-NEXT: [[TMP13:%.*]] = extractelement <4 x i1> [[TMP7]], i32 2
-; CHECK-NEXT: br i1 [[TMP13]], label [[PRED_STORE_IF3:%.*]], label [[PRED_STORE_CONTINUE4:%.*]]
-; CHECK: pred.store.if3:
-; CHECK-NEXT: [[TMP14:%.*]] = add i64 [[INDEX]], 2
-; CHECK-NEXT: [[TMP15:%.*]] = getelementptr inbounds i32, ptr [[A]], i64 [[TMP14]]
-; CHECK-NEXT: store i32 1, ptr [[TMP15]], align 4
-; CHECK-NEXT: br label [[PRED_STORE_CONTINUE4]]
-; CHECK: pred.store.continue4:
-; CHECK-NEXT: [[TMP16:%.*]] = extractelement <4 x i1> [[TMP7]], i32 3
-; CHECK-NEXT: br i1 [[TMP16]], label [[PRED_STORE_IF5:%.*]], label [[PRED_STORE_CONTINUE6]]
-; CHECK: pred.store.if5:
-; CHECK-NEXT: [[TMP17:%.*]] = add i64 [[INDEX]], 3
-; CHECK-NEXT: [[TMP18:%.*]] = getelementptr inbounds i32, ptr [[A]], i64 [[TMP17]]
-; CHECK-NEXT: store i32 1, ptr [[TMP18]], align 4
-; CHECK-NEXT: br label [[PRED_STORE_CONTINUE6]]
-; CHECK: pred.store.continue6:
-; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 4
-; CHECK-NEXT: [[TMP19:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
-; CHECK-NEXT: br i1 [[TMP19]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP4:![0-9]+]]
-; CHECK: middle.block:
-; CHECK-NEXT: br label [[SCALAR_PH]]
-; CHECK: scalar.ph:
-; CHECK-NEXT: [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
; CHECK-NEXT: br label [[LOOP_HEADER:%.*]]
; CHECK: loop.header:
-; CHECK-NEXT: [[IV:%.*]] = phi i64 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
+; CHECK-NEXT: [[IV:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
; CHECK-NEXT: [[GEP:%.*]] = getelementptr inbounds i32, ptr [[A]], i64 [[IV]]
; CHECK-NEXT: [[L:%.*]] = load i32, ptr [[GEP]], align 4
; CHECK-NEXT: [[C_2:%.*]] = icmp eq i32 [[L]], 10
@@ -148,7 +91,7 @@ define i64 @multi_exit_2_exit_count_with_udiv_in_block_executed_unconditionally(
; CHECK: loop.latch:
; CHECK-NEXT: [[IV_NEXT]] = add i64 [[IV]], 1
; CHECK-NEXT: [[C_0:%.*]] = icmp slt i64 [[IV]], [[N]]
-; CHECK-NEXT: br i1 [[C_0]], label [[LOOP_HEADER]], label [[EXIT]], !llvm.loop [[LOOP5:![0-9]+]]
+; CHECK-NEXT: br i1 [[C_0]], label [[LOOP_HEADER]], label [[EXIT]]
; CHECK: exit:
; CHECK-NEXT: [[P:%.*]] = phi i64 [ 1, [[CONTINUE]] ], [ 0, [[LOOP_LATCH]] ]
; CHECK-NEXT: ret i64 [[P]]
@@ -232,40 +175,13 @@ exit:
ret i64 %p
}
-; FIXME: Currently miscompiled as we unconditionally execute udiv after
-; vectorization.
define i64 @multi_exit_4_exit_count_with_udiv_in_latch(ptr %dst, i64 %N) {
; CHECK-LABEL: define i64 @multi_exit_4_exit_count_with_udiv_in_latch(
; CHECK-SAME: ptr [[DST:%.*]], i64 [[N:%.*]]) {
; CHECK-NEXT: entry:
-; CHECK-NEXT: [[SMAX:%.*]] = call i64 @llvm.smax.i64(i64 [[N]], i64 0)
-; CHECK-NEXT: [[TMP0:%.*]] = udiv i64 42, [[N]]
-; CHECK-NEXT: [[UMIN:%.*]] = call i64 @llvm.umin.i64(i64 [[SMAX]], i64 [[TMP0]])
-; CHECK-NEXT: [[TMP1:%.*]] = add nuw nsw i64 [[UMIN]], 1
-; CHECK-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ule i64 [[TMP1]], 4
-; CHECK-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
-; CHECK: vector.ph:
-; CHECK-NEXT: [[N_MOD_VF:%.*]] = urem i64 [[TMP1]], 4
-; CHECK-NEXT: [[TMP2:%.*]] = icmp eq i64 [[N_MOD_VF]], 0
-; CHECK-NEXT: [[TMP3:%.*]] = select i1 [[TMP2]], i64 4, i64 [[N_MOD_VF]]
-; CHECK-NEXT: [[N_VEC:%.*]] = sub i64 [[TMP1]], [[TMP3]]
-; CHECK-NEXT: br label [[VECTOR_BODY:%.*]]
-; CHECK: vector.body:
-; CHECK-NEXT: [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
-; CHECK-NEXT: [[TMP4:%.*]] = add i64 [[INDEX]], 0
-; CHECK-NEXT: [[TMP5:%.*]] = getelementptr inbounds i32, ptr [[DST]], i64 [[TMP4]]
-; CHECK-NEXT: [[TMP6:%.*]] = getelementptr inbounds i32, ptr [[TMP5]], i32 0
-; CHECK-NEXT: store <4 x i32> <i32 1, i32 1, i32 1, i32 1>, ptr [[TMP6]], align 4
-; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 4
-; CHECK-NEXT: [[TMP7:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
-; CHECK-NEXT: br i1 [[TMP7]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP6:![0-9]+]]
-; CHECK: middle.block:
-; CHECK-NEXT: br label [[SCALAR_PH]]
-; CHECK: scalar.ph:
-; CHECK-NEXT: [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
; CHECK-NEXT: br label [[LOOP_HEADER:%.*]]
; CHECK: loop.header:
-; CHECK-NEXT: [[IV:%.*]] = phi i64 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
+; CHECK-NEXT: [[IV:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
; CHECK-NEXT: [[GEP:%.*]] = getelementptr inbounds i32, ptr [[DST]], i64 [[IV]]
; CHECK-NEXT: store i32 1, ptr [[GEP]], align 4
; CHECK-NEXT: [[C_0:%.*]] = icmp slt i64 [[IV]], [[N]]
@@ -274,7 +190,7 @@ define i64 @multi_exit_4_exit_count_with_udiv_in_latch(ptr %dst, i64 %N) {
; CHECK-NEXT: [[IV_NEXT]] = add i64 [[IV]], 1
; CHECK-NEXT: [[D:%.*]] = udiv i64 42, [[N]]
; CHECK-NEXT: [[C_1:%.*]] = icmp slt i64 [[IV]], [[D]]
-; CHECK-NEXT: br i1 [[C_1]], label [[LOOP_HEADER]], label [[EXIT]], !llvm.loop [[LOOP7:![0-9]+]]
+; CHECK-NEXT: br i1 [[C_1]], label [[LOOP_HEADER]], label [[EXIT]]
; CHECK: exit:
; CHECK-NEXT: [[P:%.*]] = phi i64 [ 1, [[LOOP_HEADER]] ], [ 0, [[LOOP_LATCH]] ]
; CHECK-NEXT: ret i64 [[P]]
@@ -320,7 +236,7 @@ define void @single_exit_tc_with_udiv(ptr %dst, i64 %N) {
; CHECK-NEXT: store <4 x i32> <i32 1, i32 1, i32 1, i32 1>, ptr [[TMP4]], align 4
; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 4
; CHECK-NEXT: [[TMP5:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
-; CHECK-NEXT: br i1 [[TMP5]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP8:![0-9]+]]
+; CHECK-NEXT: br i1 [[TMP5]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP4:![0-9]+]]
; CHECK: middle.block:
; CHECK-NEXT: [[CMP_N:%.*]] = icmp eq i64 [[TMP1]], [[N_VEC]]
; CHECK-NEXT: br i1 [[CMP_N]], label [[EXIT:%.*]], label [[SCALAR_PH]]
@@ -334,7 +250,7 @@ define void @single_exit_tc_with_udiv(ptr %dst, i64 %N) {
; CHECK-NEXT: [[IV_NEXT]] = add i64 [[IV]], 1
; CHECK-NEXT: [[D:%.*]] = udiv i64 42, [[N]]
; CHECK-NEXT: [[C_1:%.*]] = icmp slt i64 [[IV]], [[D]]
-; CHECK-NEXT: br i1 [[C_1]], label [[LOOP]], label [[EXIT]], !llvm.loop [[LOOP9:![0-9]+]]
+; CHECK-NEXT: br i1 [[C_1]], label [[LOOP]], label [[EXIT]], !llvm.loop [[LOOP5:![0-9]+]]
; CHECK: exit:
; CHECK-NEXT: ret void
;
@@ -361,8 +277,4 @@ exit:
; CHECK: [[LOOP3]] = distinct !{[[LOOP3]], [[META2]], [[META1]]}
; CHECK: [[LOOP4]] = distinct !{[[LOOP4]], [[META1]], [[META2]]}
; CHECK: [[LOOP5]] = distinct !{[[LOOP5]], [[META2]], [[META1]]}
-; CHECK: [[LOOP6]] = distinct !{[[LOOP6]], [[META1]], [[META2]]}
-; CHECK: [[LOOP7]] = distinct !{[[LOOP7]], [[META2]], [[META1]]}
-; CHECK: [[LOOP8]] = distinct !{[[LOOP8]], [[META1]], [[META2]]}
-; CHECK: [[LOOP9]] = distinct !{[[LOOP9]], [[META2]], [[META1]]}
;.
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