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[SimplifyCFG] avoid illegal phi with both poison and undef

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In the example based on:
https://llvm.org/PR49218
...we are crashing because poison is a subclass of undef, so we merge blocks and create:

PHI node has multiple entries for the same basic block with different incoming values!
  %k3 = phi i64 [ poison, %entry ], [ %k3, %g ], [ undef, %entry ]

If both poison and undef values are incoming, we soften the poison values to undef.

Differential Revision: https://reviews.llvm.org/D97495
This commit is contained in:
Sanjay Patel 2021-02-27 09:09:03 -05:00
parent 80c4b80982
commit c42cc1f879
2 changed files with 223 additions and 1 deletions
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24
lib/Transforms/Utils/Local.cpp
View File

@ -918,6 +918,7 @@ static void gatherIncomingValuesToPhi(PHINode *PN,
/// \param IncomingValues A map from block to value.
static void replaceUndefValuesInPhi(PHINode *PN,
const IncomingValueMap &IncomingValues) {
SmallVector<unsigned> TrueUndefOps;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
Value *V = PN->getIncomingValue(i);
@ -925,10 +926,31 @@ static void replaceUndefValuesInPhi(PHINode *PN,
BasicBlock *BB = PN->getIncomingBlock(i);
IncomingValueMap::const_iterator It = IncomingValues.find(BB);
if (It == IncomingValues.end()) continue;
// Keep track of undef/poison incoming values. Those must match, so we fix
// them up below if needed.
// Note: this is conservatively correct, but we could try harder and group
// the undef values per incoming basic block.
if (It == IncomingValues.end()) {
TrueUndefOps.push_back(i);
continue;
}
// There is a defined value for this incoming block, so map this undef
// incoming value to the defined value.
PN->setIncomingValue(i, It->second);
}
// If there are both undef and poison values incoming, then convert those
// values to undef. It is invalid to have different values for the same
// incoming block.
unsigned PoisonCount = count_if(TrueUndefOps, [&](unsigned i) {
return isa<PoisonValue>(PN->getIncomingValue(i));
});
if (PoisonCount != 0 && PoisonCount != TrueUndefOps.size()) {
for (unsigned i : TrueUndefOps)
PN->setIncomingValue(i, UndefValue::get(PN->getType()));
}
}
/// Replace a value flowing from a block to a phi with

200
test/Transforms/SimplifyCFG/poison-merge.ll Normal file
View File

@ -0,0 +1,200 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt -S -simplifycfg -simplifycfg-require-and-preserve-domtree=1 -keep-loops=0 < %s | FileCheck %s
; Merge 2 undefined incoming values.
define i32 @undef_merge(i32 %x) {
; CHECK-LABEL: @undef_merge(
; CHECK-NEXT: entry:
; CHECK-NEXT: switch i32 [[X:%.*]], label [[EXIT:%.*]] [
; CHECK-NEXT: i32 4, label [[G:%.*]]
; CHECK-NEXT: i32 12, label [[G]]
; CHECK-NEXT: ]
; CHECK: g:
; CHECK-NEXT: [[K3:%.*]] = phi i64 [ undef, [[ENTRY:%.*]] ], [ [[K3]], [[G]] ], [ undef, [[ENTRY]] ]
; CHECK-NEXT: br label [[G]]
; CHECK: exit:
; CHECK-NEXT: ret i32 undef
;
entry:
switch i32 %x, label %exit [
i32 4, label %loop
i32 12, label %g
]
loop:
%k2 = phi i64 [ %k3, %g ], [ undef, %entry ]
br label %g
g:
%k3 = phi i64 [ %k2, %loop ], [ undef, %entry ]
br label %loop
exit:
ret i32 undef
}
; Merge 2 poison incoming values.
define i32 @poison_merge(i32 %x) {
; CHECK-LABEL: @poison_merge(
; CHECK-NEXT: entry:
; CHECK-NEXT: switch i32 [[X:%.*]], label [[EXIT:%.*]] [
; CHECK-NEXT: i32 4, label [[G:%.*]]
; CHECK-NEXT: i32 12, label [[G]]
; CHECK-NEXT: ]
; CHECK: g:
; CHECK-NEXT: [[K3:%.*]] = phi i64 [ poison, [[ENTRY:%.*]] ], [ [[K3]], [[G]] ], [ poison, [[ENTRY]] ]
; CHECK-NEXT: br label [[G]]
; CHECK: exit:
; CHECK-NEXT: ret i32 undef
;
entry:
switch i32 %x, label %exit [
i32 4, label %loop
i32 12, label %g
]
loop:
%k2 = phi i64 [ %k3, %g ], [ poison, %entry ]
br label %g
g:
%k3 = phi i64 [ %k2, %loop ], [ poison, %entry ]
br label %loop
exit:
ret i32 undef
}
; Merge equal defined incoming values.
define i32 @defined_merge(i32 %x) {
; CHECK-LABEL: @defined_merge(
; CHECK-NEXT: entry:
; CHECK-NEXT: switch i32 [[X:%.*]], label [[EXIT:%.*]] [
; CHECK-NEXT: i32 4, label [[G:%.*]]
; CHECK-NEXT: i32 12, label [[G]]
; CHECK-NEXT: ]
; CHECK: g:
; CHECK-NEXT: [[K3:%.*]] = phi i64 [ 42, [[ENTRY:%.*]] ], [ [[K3]], [[G]] ], [ 42, [[ENTRY]] ]
; CHECK-NEXT: br label [[G]]
; CHECK: exit:
; CHECK-NEXT: ret i32 undef
;
entry:
switch i32 %x, label %exit [
i32 4, label %loop
i32 12, label %g
]
loop:
%k2 = phi i64 [ %k3, %g ], [ 42, %entry ]
br label %g
g:
%k3 = phi i64 [ %k2, %loop ], [ 42, %entry ]
br label %loop
exit:
ret i32 undef
}
; Merge defined and undef incoming values.
define i32 @defined_and_undef_merge(i32 %x) {
; CHECK-LABEL: @defined_and_undef_merge(
; CHECK-NEXT: entry:
; CHECK-NEXT: switch i32 [[X:%.*]], label [[EXIT:%.*]] [
; CHECK-NEXT: i32 4, label [[G:%.*]]
; CHECK-NEXT: i32 12, label [[G]]
; CHECK-NEXT: ]
; CHECK: g:
; CHECK-NEXT: [[K3:%.*]] = phi i64 [ 42, [[ENTRY:%.*]] ], [ [[K3]], [[G]] ], [ 42, [[ENTRY]] ]
; CHECK-NEXT: br label [[G]]
; CHECK: exit:
; CHECK-NEXT: ret i32 undef
;
entry:
switch i32 %x, label %exit [
i32 4, label %loop
i32 12, label %g
]
loop:
%k2 = phi i64 [ %k3, %g ], [ undef, %entry ]
br label %g
g:
%k3 = phi i64 [ %k2, %loop ], [ 42, %entry ]
br label %loop
exit:
ret i32 undef
}
; Merge defined and poison incoming values.
define i32 @defined_and_poison_merge(i32 %x) {
; CHECK-LABEL: @defined_and_poison_merge(
; CHECK-NEXT: entry:
; CHECK-NEXT: switch i32 [[X:%.*]], label [[EXIT:%.*]] [
; CHECK-NEXT: i32 4, label [[G:%.*]]
; CHECK-NEXT: i32 12, label [[G]]
; CHECK-NEXT: ]
; CHECK: g:
; CHECK-NEXT: [[K3:%.*]] = phi i64 [ 42, [[ENTRY:%.*]] ], [ [[K3]], [[G]] ], [ 42, [[ENTRY]] ]
; CHECK-NEXT: br label [[G]]
; CHECK: exit:
; CHECK-NEXT: ret i32 undef
;
entry:
switch i32 %x, label %exit [
i32 4, label %loop
i32 12, label %g
]
loop:
%k2 = phi i64 [ %k3, %g ], [ poison, %entry ]
br label %g
g:
%k3 = phi i64 [ %k2, %loop ], [ 42, %entry ]
br label %loop
exit:
ret i32 undef
}
; Do not crash trying to merge poison and undef into a single phi.
define i32 @PR49218(i32 %x) {
; CHECK-LABEL: @PR49218(
; CHECK-NEXT: entry:
; CHECK-NEXT: switch i32 [[X:%.*]], label [[EXIT:%.*]] [
; CHECK-NEXT: i32 4, label [[G:%.*]]
; CHECK-NEXT: i32 12, label [[G]]
; CHECK-NEXT: ]
; CHECK: g:
; CHECK-NEXT: [[K3:%.*]] = phi i64 [ undef, [[ENTRY:%.*]] ], [ [[K3]], [[G]] ], [ undef, [[ENTRY]] ]
; CHECK-NEXT: br label [[G]]
; CHECK: exit:
; CHECK-NEXT: ret i32 undef
;
entry:
switch i32 %x, label %exit [
i32 4, label %loop
i32 12, label %g
]
loop:
%k2 = phi i64 [ %k3, %g ], [ undef, %entry ]
br label %g
g:
%k3 = phi i64 [ %k2, %loop ], [ poison, %entry ]
br label %loop
exit:
ret i32 undef
}