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[SCEV] Use backedge SCEV of PHI only if its input is loop invariant

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For the PHI node

      %1 = phi [%A, %entry], [%X, %latch]

it is incorrect to use SCEV of backedge val %X as an exit value
of PHI unless %X is loop invariant.
This is because exit value of %1 is value of %X at one-before-last
iteration of the loop.

Reviewed By: Meinersbur
Differential Revision: https://reviews.llvm.org/D73181
This commit is contained in:
Denis Antrushin 2020-01-22 20:40:32 +07:00
parent 6e110faf7b
commit dcb42b6172
2 changed files with 71 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
lib/Analysis/ScalarEvolution.cpp
View File

@ -8285,10 +8285,11 @@ const SCEV *ScalarEvolution::computeSCEVAtScope(const SCEV *V, const Loop *L) {
if (!isa<SCEVCouldNotCompute>(BackedgeTakenCount) &&
isKnownPositive(BackedgeTakenCount) &&
PN->getNumIncomingValues() == 2) {
unsigned InLoopPred = LI->contains(PN->getIncomingBlock(0)) ? 0 : 1;
const SCEV *OnBackedge = getSCEV(PN->getIncomingValue(InLoopPred));
if (IsAvailableOnEntry(LI, DT, OnBackedge, PN->getParent()))
return OnBackedge;
Value *BackedgeVal = PN->getIncomingValue(InLoopPred);
if (LI->isLoopInvariant(BackedgeVal))
return getSCEV(BackedgeVal);
}
if (auto *BTCC = dyn_cast<SCEVConstant>(BackedgeTakenCount)) {
// Okay, we know how many times the containing loop executes. If

67
test/Analysis/ScalarEvolution/pr44605.ll Normal file
View File

@ -0,0 +1,67 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; NOTE: Only %local_3_4 is important here.
; All other instructions are needed to lure LLVM into executing
; specific code to trigger a bug.
; RUN: opt < %s -indvars -S | FileCheck %s
define i32 @test() {
; CHECK-LABEL: @test(
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[OUTER:%.*]]
; CHECK: outer:
; CHECK-NEXT: [[LOCAL_6_6:%.*]] = phi i32 [ 10, [[ENTRY:%.*]] ], [ [[TMP5:%.*]], [[LATCH:%.*]] ]
; CHECK-NEXT: [[LOCAL_4_5:%.*]] = phi i32 [ 56587, [[ENTRY]] ], [ 0, [[LATCH]] ]
; CHECK-NEXT: [[LOCAL_3_4:%.*]] = phi i32 [ 2, [[ENTRY]] ], [ [[TMP5]], [[LATCH]] ]
; CHECK-NEXT: [[DOTUDIV:%.*]] = udiv i32 [[LOCAL_6_6]], 8361
; CHECK-NEXT: br label [[INNER:%.*]]
; CHECK: inner:
; CHECK-NEXT: [[LOCAL_7_3:%.*]] = phi i32 [ 2, [[OUTER]] ], [ [[TMP3:%.*]], [[INNER]] ]
; CHECK-NEXT: [[LOCAL_4_5_PN:%.*]] = phi i32 [ [[LOCAL_4_5]], [[OUTER]] ], [ [[TMP2:%.*]], [[INNER]] ]
; CHECK-NEXT: [[LOCAL_3_31:%.*]] = mul i32 [[LOCAL_4_5_PN]], [[DOTUDIV]]
; CHECK-NEXT: [[TMP0:%.*]] = mul nuw nsw i32 [[LOCAL_7_3]], [[DOTUDIV]]
; CHECK-NEXT: [[TMP1:%.*]] = sub i32 [[TMP0]], [[LOCAL_3_4]]
; CHECK-NEXT: [[TMP2]] = add i32 [[TMP1]], [[LOCAL_3_31]]
; CHECK-NEXT: [[TMP3]] = add nuw nsw i32 [[LOCAL_7_3]], 1
; CHECK-NEXT: [[TMP4:%.*]] = icmp ugt i32 [[LOCAL_7_3]], 4
; CHECK-NEXT: br i1 [[TMP4]], label [[LATCH]], label [[INNER]]
; CHECK: latch:
; CHECK-NEXT: [[DOTLCSSA:%.*]] = phi i32 [ [[TMP2]], [[INNER]] ]
; CHECK-NEXT: [[TMP5]] = add nuw nsw i32 [[LOCAL_6_6]], 1
; CHECK-NEXT: [[TMP6:%.*]] = icmp ugt i32 [[LOCAL_6_6]], 276
; CHECK-NEXT: br i1 [[TMP6]], label [[RETURN:%.*]], label [[OUTER]]
; CHECK: return:
; CHECK-NEXT: [[DOTLCSSA_LCSSA:%.*]] = phi i32 [ [[DOTLCSSA]], [[LATCH]] ]
; CHECK-NEXT: ret i32 [[DOTLCSSA_LCSSA]]
;
entry:
br label %outer
outer:
%local_6_6 = phi i32 [ 10, %entry ], [ %5, %latch ]
%local_4_5 = phi i32 [ 56587, %entry ], [ 0, %latch ]
%local_3_4 = phi i32 [ 2, %entry ], [ %5, %latch ]
%.udiv = udiv i32 %local_6_6, 8361
br label %inner
inner:
%local_7_3 = phi i32 [ 2, %outer ], [ %3, %inner ]
%local_4_5.pn = phi i32 [ %local_4_5, %outer ], [ %2, %inner ]
%local_3_31 = mul i32 %local_4_5.pn, %.udiv
%0 = mul i32 %local_7_3, %.udiv
%1 = sub i32 %0, %local_3_4
%2 = add i32 %1, %local_3_31
%3 = add nuw nsw i32 %local_7_3, 1
%4 = icmp ugt i32 %local_7_3, 4
br i1 %4, label %latch, label %inner
latch:
%.lcssa = phi i32 [ %2, %inner ]
%5 = add nuw nsw i32 %local_6_6, 1
%6 = icmp ugt i32 %local_6_6, 276
br i1 %6, label %return, label %outer
return:
%.lcssa.lcssa = phi i32 [ %.lcssa, %latch ]
ret i32 %.lcssa.lcssa
}