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[LCSSA] Forget values we create LCSSA phis for 

Summary:
Currently we only forget the loop we added LCSSA phis for. But SCEV
expressions in other loops could also depend on the instruction we added
a PHI for and currently we do not invalidate those expressions. This can
happen when we use ScalarEvolution before converting a function to LCSSA
form. The SCEV expressions will refer to the non-LCSSA value. If this
SCEV expression is then used with the expander, we do not preserve LCSSA
form.

This patch properly forgets the values we created PHIs for. Those need
to be recomputed again. This patch fixes PR43458.

Currently SCEV::verify does not catch this mismatch and any test would
need to run multiple passes to trigger the error (e.g. -loop-reduce
-loop-unroll). I will also look into catching this kind of mismatch in
the verifier. Also, we currently forget the whole loop in LCSSA and I'll
check if we can be more surgical.

Reviewers: efriedma, sanjoy.google, reames

Reviewed By: efriedma

Subscribers: zzheng, hiraditya, javed.absar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68194
This commit is contained in:
Florian Hahn 2019-10-29 11:54:18 +00:00
parent 2c496bb530
commit 596e4ab97a
3 changed files with 81 additions and 3 deletions
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3
llvm/include/llvm/Transforms/Utils/LoopUtils.h
View File

@ -73,7 +73,8 @@ bool formDedicatedExitBlocks(Loop *L, DominatorTree *DT, LoopInfo *LI,
///
/// Returns true if any modifications are made.
bool formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
DominatorTree &DT, LoopInfo &LI);
DominatorTree &DT, LoopInfo &LI,
ScalarEvolution *SE);
/// Put loop into LCSSA form.
///

10
llvm/lib/Transforms/Utils/LCSSA.cpp
View File

@ -74,7 +74,8 @@ static bool isExitBlock(BasicBlock *BB,
/// that are outside the current loop. If so, insert LCSSA PHI nodes and
/// rewrite the uses.
bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
DominatorTree &DT, LoopInfo &LI) {
DominatorTree &DT, LoopInfo &LI,
ScalarEvolution *SE) {
SmallVector<Use *, 16> UsesToRewrite;
SmallSetVector<PHINode *, 16> PHIsToRemove;
PredIteratorCache PredCache;
@ -134,6 +135,11 @@ bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
SSAUpdater SSAUpdate(&InsertedPHIs);
SSAUpdate.Initialize(I->getType(), I->getName());
// Force re-computation of I, as some users now need to use the new PHI
// node.
if (SE)
SE->forgetValue(I);
// Insert the LCSSA phi's into all of the exit blocks dominated by the
// value, and add them to the Phi's map.
for (BasicBlock *ExitBB : ExitBlocks) {
@ -368,7 +374,7 @@ bool llvm::formLCSSA(Loop &L, DominatorTree &DT, LoopInfo *LI,
Worklist.push_back(&I);
}
}
Changed = formLCSSAForInstructions(Worklist, DT, *LI);
Changed = formLCSSAForInstructions(Worklist, DT, *LI, SE);
// If we modified the code, remove any caches about the loop from SCEV to
// avoid dangling entries.

71
llvm/test/Transforms/LoopUnroll/unroll-preserve-scev-lcssa.ll Normal file
View File

@ -0,0 +1,71 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt -loop-reduce -loop-unroll -unroll-runtime -verify-scev -verify-loop-lcssa < %s -S | FileCheck %s
; Test case for PR43458. We check that we do properly invalidate LCSSA phi
; users in SCEV. Run -loop-reduce first, so SCEV is already populated.
define void @spam() {
; CHECK-LABEL: @spam(
; CHECK-NEXT: bb:
; CHECK-NEXT: br label [[BB3:%.*]]
; CHECK: bb3:
; CHECK-NEXT: br i1 false, label [[BB9:%.*]], label [[BB5_PREHEADER:%.*]]
; CHECK: bb5.preheader:
; CHECK-NEXT: br label [[BB5:%.*]]
; CHECK: bb5:
; CHECK-NEXT: br label [[BB9]]
; CHECK: bb9:
; CHECK-NEXT: [[TMP10:%.*]] = phi i64 [ 0, [[BB3]] ], [ 5, [[BB5]] ]
; CHECK-NEXT: [[TMP11:%.*]] = trunc i64 [[TMP10]] to i32
; CHECK-NEXT: [[TMP21:%.*]] = sext i32 [[TMP11]] to i64
; CHECK-NEXT: [[TMP22:%.*]] = icmp slt i64 0, [[TMP21]]
; CHECK-NEXT: br i1 [[TMP22]], label [[BB24_PREHEADER:%.*]], label [[BB29:%.*]]
; CHECK: bb24.preheader:
; CHECK-NEXT: br label [[BB24:%.*]]
; CHECK: bb24:
; CHECK-NEXT: [[TMP25:%.*]] = phi i64 [ [[TMP26:%.*]], [[BB24]] ], [ 0, [[BB24_PREHEADER]] ]
; CHECK-NEXT: [[TMP26]] = add nuw nsw i64 [[TMP25]], 1
; CHECK-NEXT: [[TMP27:%.*]] = icmp slt i64 [[TMP26]], [[TMP21]]
; CHECK-NEXT: br i1 [[TMP27]], label [[BB24]], label [[BB29_LOOPEXIT:%.*]]
; CHECK: bb29.loopexit:
; CHECK-NEXT: br label [[BB29]]
; CHECK: bb29:
; CHECK-NEXT: ret void
;
bb:
br label %bb3
bb3: ; preds = %bb9, %bb
%pv1 = phi i64 [ 0, %bb ], [ %iv1, %bb9]
%c1 = icmp eq i64 %pv1, 5
br i1 %c1, label %bb9, label %bb5
bb5: ; preds = %bb5, %bb3
%pv = phi i64 [ 0, %bb3], [ %tmp6, %bb5]
%tmp6 = add nsw i64 %pv, 1
%cond = icmp eq i64 %tmp6, 5
br i1 %cond, label %bb8, label %bb5
bb8: ; preds = %bb5
br label %bb9
bb9: ; preds = %bb8, %bb3
%tmp10 = phi i64 [ 0, %bb3 ], [ %tmp6, %bb8 ]
%tmp11 = trunc i64 %tmp10 to i32
%iv1 = add nsw i64 %pv1, 1
br i1 false, label %bb3, label %bb20
bb20: ; preds = %bb9
%tmp21 = sext i32 %tmp11 to i64
%tmp22 = icmp slt i64 0, %tmp21
br i1 %tmp22, label %bb24, label %bb29
bb24: ; preds = %bb24, %bb20
%tmp25 = phi i64 [ %tmp26, %bb24 ], [ 0, %bb20 ]
%tmp26 = add nuw nsw i64 %tmp25, 1
%tmp27 = icmp slt i64 %tmp26, %tmp21
br i1 %tmp27, label %bb24, label %bb29
bb29: ; preds = %bb24, %bb20
ret void
}