[llvm] f7ef072 - [SCEV] Do not allow refinement in the rewriting of BEValue (#117152)
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Sun Dec 1 04:11:13 PST 2024
Author: Yingwei Zheng
Date: 2024-12-01T20:11:09+08:00
New Revision: f7ef0721d60f85e1f699f8d1b83d4402ae19b122
URL: https://github.com/llvm/llvm-project/commit/f7ef0721d60f85e1f699f8d1b83d4402ae19b122
DIFF: https://github.com/llvm/llvm-project/commit/f7ef0721d60f85e1f699f8d1b83d4402ae19b122.diff
LOG: [SCEV] Do not allow refinement in the rewriting of BEValue (#117152)
See the following case:
```
; bin/opt -passes="print<scalar-evolution>" test.ll --disable-output
define i32 @widget() {
b:
br label %b1
b1: ; preds = %b5, %b
%phi = phi i32 [ 0, %b ], [ %udiv6, %b5 ]
%phi2 = phi i32 [ 1, %b ], [ %add, %b5 ]
%icmp = icmp eq i32 %phi, 0
br i1 %icmp, label %b3, label %b8
b3: ; preds = %b1
%udiv = udiv i32 10, %phi2
%urem = urem i32 %udiv, 10
%icmp4 = icmp eq i32 %urem, 0
br i1 %icmp4, label %b7, label %b5
b5: ; preds = %b3
%udiv6 = udiv i32 %phi2, 0
%add = add i32 %phi2, 1
br label %b1
b7: ; preds = %b3
ret i32 5
b8: ; preds = %b1
ret i32 7
}
```
```
%phi2 = phi i32 [ 1, %b ], [ %add, %b5 ] --> {1,+,1}<nuw><nsw><%b1>
%udiv6 = udiv i32 %phi2, 0 --> ({1,+,1}<nuw><nsw><%b1> /u 0)
%phi = phi i32 [ 0, %b ], [ %udiv6, %b5 ] --> ({0,+,1}<nuw><nsw><%b1> /u 0)
```
`ScalarEvolution::createAddRecFromPHI` gives a wrong SCEV result for
`%phi`:
https://github.com/llvm/llvm-project/blob/d7d6fb1804415b0f3e7f1cc9290bfb3d711cb707/llvm/lib/Analysis/ScalarEvolution.cpp#L5926-L5950
It converts `phi(0, ({1,+,1}<nuw><nsw><%b1> /u 0))` into `phi(0 / 0,
({1,+,1}<nuw><nsw><%b1> /u 0))`. Then it simplifies the expr into
`{0,+,1}<nuw><nsw><%b1> /u 0`.
As we did in
https://github.com/llvm/llvm-project/commit/acd700a24b6f767413db3d525e06d03e4245aa40,
this patch disallows udiv simplification if we cannot prove that the
denominator is a well-defined non-zero value.
Fixes https://github.com/llvm/llvm-project/issues/117133.
Added:
llvm/test/Analysis/ScalarEvolution/pr117133.ll
llvm/test/Transforms/IndVarSimplify/pr117133.ll
Modified:
llvm/include/llvm/Analysis/ScalarEvolution.h
llvm/lib/Analysis/ScalarEvolution.cpp
Removed:
################################################################################
diff --git a/llvm/include/llvm/Analysis/ScalarEvolution.h b/llvm/include/llvm/Analysis/ScalarEvolution.h
index 885c5985f9d23a..b20c6a13cb6bd7 100644
--- a/llvm/include/llvm/Analysis/ScalarEvolution.h
+++ b/llvm/include/llvm/Analysis/ScalarEvolution.h
@@ -2187,6 +2187,9 @@ class ScalarEvolution {
bool isGuaranteedToTransferExecutionTo(const Instruction *A,
const Instruction *B);
+ /// Returns true if \p Op is guaranteed not to cause immediate UB.
+ bool isGuaranteedNotToCauseUB(const SCEV *Op);
+
/// Returns true if \p Op is guaranteed to not be poison.
static bool isGuaranteedNotToBePoison(const SCEV *Op);
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index c3f296b9ff3347..882e938e69c0c2 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -4307,15 +4307,7 @@ ScalarEvolution::getSequentialMinMaxExpr(SCEVTypes Kind,
}
for (unsigned i = 1, e = Ops.size(); i != e; ++i) {
- bool MayBeUB = SCEVExprContains(Ops[i], [this](const SCEV *S) {
- auto *UDiv = dyn_cast<SCEVUDivExpr>(S);
- // The UDiv may be UB if the divisor is poison or zero. Unless the divisor
- // is a non-zero constant, we have to assume the UDiv may be UB.
- return UDiv && (!isKnownNonZero(UDiv->getOperand(1)) ||
- !isGuaranteedNotToBePoison(UDiv->getOperand(1)));
- });
-
- if (MayBeUB)
+ if (!isGuaranteedNotToCauseUB(Ops[i]))
continue;
// We can replace %x umin_seq %y with %x umin %y if either:
// * %y being poison implies %x is also poison.
@@ -5933,18 +5925,22 @@ const SCEV *ScalarEvolution::createAddRecFromPHI(PHINode *PN) {
// We can generalize this saying that i is the shifted value of BEValue
// by one iteration:
// PHI(f(0), f({1,+,1})) --> f({0,+,1})
- const SCEV *Shifted = SCEVShiftRewriter::rewrite(BEValue, L, *this);
- const SCEV *Start = SCEVInitRewriter::rewrite(Shifted, L, *this, false);
- if (Shifted != getCouldNotCompute() &&
- Start != getCouldNotCompute()) {
- const SCEV *StartVal = getSCEV(StartValueV);
- if (Start == StartVal) {
- // Okay, for the entire analysis of this edge we assumed the PHI
- // to be symbolic. We now need to go back and purge all of the
- // entries for the scalars that use the symbolic expression.
- forgetMemoizedResults(SymbolicName);
- insertValueToMap(PN, Shifted);
- return Shifted;
+
+ // Do not allow refinement in rewriting of BEValue.
+ if (isGuaranteedNotToCauseUB(BEValue)) {
+ const SCEV *Shifted = SCEVShiftRewriter::rewrite(BEValue, L, *this);
+ const SCEV *Start = SCEVInitRewriter::rewrite(Shifted, L, *this, false);
+ if (Shifted != getCouldNotCompute() && Start != getCouldNotCompute() &&
+ ::impliesPoison(BEValue, Start)) {
+ const SCEV *StartVal = getSCEV(StartValueV);
+ if (Start == StartVal) {
+ // Okay, for the entire analysis of this edge we assumed the PHI
+ // to be symbolic. We now need to go back and purge all of the
+ // entries for the scalars that use the symbolic expression.
+ forgetMemoizedResults(SymbolicName);
+ insertValueToMap(PN, Shifted);
+ return Shifted;
+ }
}
}
}
@@ -7324,6 +7320,16 @@ bool ScalarEvolution::isGuaranteedNotToBePoison(const SCEV *Op) {
return PC.MaybePoison.empty();
}
+bool ScalarEvolution::isGuaranteedNotToCauseUB(const SCEV *Op) {
+ return !SCEVExprContains(Op, [this](const SCEV *S) {
+ auto *UDiv = dyn_cast<SCEVUDivExpr>(S);
+ // The UDiv may be UB if the divisor is poison or zero. Unless the divisor
+ // is a non-zero constant, we have to assume the UDiv may be UB.
+ return UDiv && (!isKnownNonZero(UDiv->getOperand(1)) ||
+ !isGuaranteedNotToBePoison(UDiv->getOperand(1)));
+ });
+}
+
bool ScalarEvolution::isSCEVExprNeverPoison(const Instruction *I) {
// Only proceed if we can prove that I does not yield poison.
if (!programUndefinedIfPoison(I))
diff --git a/llvm/test/Analysis/ScalarEvolution/pr117133.ll b/llvm/test/Analysis/ScalarEvolution/pr117133.ll
new file mode 100644
index 00000000000000..3aadc18d6157c8
--- /dev/null
+++ b/llvm/test/Analysis/ScalarEvolution/pr117133.ll
@@ -0,0 +1,97 @@
+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -disable-output "-passes=print<scalar-evolution>" < %s 2>&1 | FileCheck %s
+
+define i32 @widget() {
+; CHECK-LABEL: 'widget'
+; CHECK-NEXT: Classifying expressions for: @widget
+; CHECK-NEXT: %phi = phi i32 [ 0, %b ], [ %udiv6, %b5 ]
+; CHECK-NEXT: --> %phi U: [0,1) S: [0,1) Exits: <<Unknown>> LoopDispositions: { %b1: Variant }
+; CHECK-NEXT: %phi2 = phi i32 [ 1, %b ], [ %add, %b5 ]
+; CHECK-NEXT: --> {1,+,1}<nuw><nsw><%b1> U: [1,2) S: [1,2) Exits: <<Unknown>> LoopDispositions: { %b1: Computable }
+; CHECK-NEXT: %udiv = udiv i32 10, %phi2
+; CHECK-NEXT: --> (10 /u {1,+,1}<nuw><nsw><%b1>) U: [10,11) S: [10,11) Exits: <<Unknown>> LoopDispositions: { %b1: Computable }
+; CHECK-NEXT: %urem = urem i32 %udiv, 10
+; CHECK-NEXT: --> ((-10 * ((10 /u {1,+,1}<nuw><nsw><%b1>) /u 10))<nuw><nsw> + (10 /u {1,+,1}<nuw><nsw><%b1>)) U: [0,1) S: [0,1) Exits: <<Unknown>> LoopDispositions: { %b1: Computable }
+; CHECK-NEXT: %udiv6 = udiv i32 %phi2, 0
+; CHECK-NEXT: --> ({1,+,1}<nuw><nsw><%b1> /u 0) U: empty-set S: empty-set Exits: <<Unknown>> LoopDispositions: { %b1: Computable }
+; CHECK-NEXT: %add = add i32 %phi2, 1
+; CHECK-NEXT: --> {2,+,1}<nuw><nsw><%b1> U: [2,3) S: [2,3) Exits: <<Unknown>> LoopDispositions: { %b1: Computable }
+; CHECK-NEXT: Determining loop execution counts for: @widget
+; CHECK-NEXT: Loop %b1: <multiple exits> Unpredictable backedge-taken count.
+; CHECK-NEXT: exit count for b1: ***COULDNOTCOMPUTE***
+; CHECK-NEXT: exit count for b3: i32 0
+; CHECK-NEXT: Loop %b1: constant max backedge-taken count is i32 0
+; CHECK-NEXT: Loop %b1: symbolic max backedge-taken count is i32 0
+; CHECK-NEXT: symbolic max exit count for b1: ***COULDNOTCOMPUTE***
+; CHECK-NEXT: symbolic max exit count for b3: i32 0
+;
+b:
+ br label %b1
+
+b1: ; preds = %b5, %b
+ %phi = phi i32 [ 0, %b ], [ %udiv6, %b5 ]
+ %phi2 = phi i32 [ 1, %b ], [ %add, %b5 ]
+ %icmp = icmp eq i32 %phi, 0
+ br i1 %icmp, label %b3, label %b8
+
+b3: ; preds = %b1
+ %udiv = udiv i32 10, %phi2
+ %urem = urem i32 %udiv, 10
+ %icmp4 = icmp eq i32 %urem, 0
+ br i1 %icmp4, label %b7, label %b5
+
+b5: ; preds = %b3
+ %udiv6 = udiv i32 %phi2, 0
+ %add = add i32 %phi2, 1
+ br label %b1
+
+b7: ; preds = %b3
+ ret i32 5
+
+b8: ; preds = %b1
+ ret i32 7
+}
+
+; Don't fold %indvar2 into (zext {0,+,1}) * %a
+define i64 @test_poisonous(i64 %a, i32 %n) {
+; CHECK-LABEL: 'test_poisonous'
+; CHECK-NEXT: Classifying expressions for: @test_poisonous
+; CHECK-NEXT: %indvar1 = phi i32 [ 0, %entry ], [ %indvar1.next, %loop.body ]
+; CHECK-NEXT: --> {0,+,1}<%loop.body> U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %loop.body: Computable }
+; CHECK-NEXT: %indvar2 = phi i64 [ 0, %entry ], [ %mul, %loop.body ]
+; CHECK-NEXT: --> %indvar2 U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %loop.body: Variant }
+; CHECK-NEXT: %indvar1.next = add i32 %indvar1, 1
+; CHECK-NEXT: --> {1,+,1}<%loop.body> U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %loop.body: Computable }
+; CHECK-NEXT: %ext = zext i32 %indvar1.next to i64
+; CHECK-NEXT: --> (zext i32 {1,+,1}<%loop.body> to i64) U: [0,4294967296) S: [0,4294967296) Exits: <<Unknown>> LoopDispositions: { %loop.body: Computable }
+; CHECK-NEXT: %mul = mul i64 %ext, %a
+; CHECK-NEXT: --> ((zext i32 {1,+,1}<%loop.body> to i64) * %a) U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %loop.body: Computable }
+; CHECK-NEXT: Determining loop execution counts for: @test_poisonous
+; CHECK-NEXT: Loop %loop.body: Unpredictable backedge-taken count.
+; CHECK-NEXT: Loop %loop.body: Unpredictable constant max backedge-taken count.
+; CHECK-NEXT: Loop %loop.body: Unpredictable symbolic max backedge-taken count.
+; CHECK-NEXT: Loop %loop.body: Predicated backedge-taken count is (-1 + (1 smax (1 + (sext i32 %n to i64))<nsw>))<nsw>
+; CHECK-NEXT: Predicates:
+; CHECK-NEXT: {1,+,1}<%loop.body> Added Flags: <nssw>
+; CHECK-NEXT: Loop %loop.body: Predicated constant max backedge-taken count is i64 2147483647
+; CHECK-NEXT: Predicates:
+; CHECK-NEXT: {1,+,1}<%loop.body> Added Flags: <nssw>
+; CHECK-NEXT: Loop %loop.body: Predicated symbolic max backedge-taken count is (-1 + (1 smax (1 + (sext i32 %n to i64))<nsw>))<nsw>
+; CHECK-NEXT: Predicates:
+; CHECK-NEXT: {1,+,1}<%loop.body> Added Flags: <nssw>
+;
+entry:
+ br label %loop.body
+
+loop.body:
+ %indvar1 = phi i32 [ 0, %entry ], [ %indvar1.next, %loop.body ]
+ %indvar2 = phi i64 [ 0, %entry ], [ %mul, %loop.body ]
+ %indvar1.next = add i32 %indvar1, 1
+ %ext = zext i32 %indvar1.next to i64
+ %mul = mul i64 %ext, %a
+ %exitcond = icmp sgt i32 %indvar1.next, %n
+ br i1 %exitcond, label %loop.exit, label %loop.body
+
+loop.exit:
+ ret i64 %mul
+}
diff --git a/llvm/test/Transforms/IndVarSimplify/pr117133.ll b/llvm/test/Transforms/IndVarSimplify/pr117133.ll
new file mode 100644
index 00000000000000..f62dc3f04a0966
--- /dev/null
+++ b/llvm/test/Transforms/IndVarSimplify/pr117133.ll
@@ -0,0 +1,44 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes=indvars < %s | FileCheck %s
+
+define i32 @widget() {
+; CHECK-LABEL: define i32 @widget() {
+; CHECK-NEXT: [[B:.*:]]
+; CHECK-NEXT: br label %[[B1:.*]]
+; CHECK: [[B1]]:
+; CHECK-NEXT: br i1 true, label %[[B3:.*]], label %[[B8:.*]]
+; CHECK: [[B3]]:
+; CHECK-NEXT: br i1 true, label %[[B7:.*]], label %[[B5:.*]]
+; CHECK: [[B5]]:
+; CHECK-NEXT: br label %[[B1]]
+; CHECK: [[B7]]:
+; CHECK-NEXT: ret i32 5
+; CHECK: [[B8]]:
+; CHECK-NEXT: ret i32 7
+;
+b:
+ br label %b1
+
+b1:
+ %phi = phi i32 [ 0, %b ], [ %udiv6, %b5 ]
+ %phi2 = phi i32 [ 1, %b ], [ %add, %b5 ]
+ %icmp = icmp eq i32 %phi, 0
+ br i1 %icmp, label %b3, label %b8
+
+b3:
+ %udiv = udiv i32 10, %phi2
+ %urem = urem i32 %udiv, 10
+ %icmp4 = icmp eq i32 %urem, 0
+ br i1 %icmp4, label %b7, label %b5
+
+b5:
+ %udiv6 = udiv i32 %phi2, 0
+ %add = add i32 %phi2, 1
+ br label %b1
+
+b7:
+ ret i32 5
+
+b8:
+ ret i32 7
+}
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