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[ValueTracking] add scan limit for assumes 

In the motivating example from https://llvm.org/PR49171 and
reduced test here, we would unroll and clone assumes so much
that compile-time effectively became infinite while analyzing
all of those assumes.
This commit is contained in:
Sanjay Patel 2021-02-15 14:53:07 -05:00
parent 328261019f
commit 378941f611
2 changed files with 38 additions and 1 deletions
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6
llvm/lib/Analysis/ValueTracking.cpp
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@ -551,8 +551,12 @@ bool llvm::isValidAssumeForContext(const Instruction *Inv,
// The context comes first, but they're both in the same block.
// Make sure there is nothing in between that might interrupt
// the control flow, not even CxtI itself.
// We limit the scan distance between the assume and its context instruction
// to avoid a compile-time explosion. This limit is chosen arbitrarily, so
// it can be adjusted if needed (could be turned into a cl::opt).
unsigned ScanLimit = 15;
for (BasicBlock::const_iterator I(CxtI), IE(Inv); I != IE; ++I)
if (!isGuaranteedToTransferExecutionToSuccessor(&*I))
if (!isGuaranteedToTransferExecutionToSuccessor(&*I) || --ScanLimit == 0)
return false;
return !isEphemeralValueOf(Inv, CxtI);

33
llvm/test/Transforms/LoopUnroll/assume-timeout.ll Normal file
View File

@ -0,0 +1,33 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt -loop-unroll -unroll-threshold=300 -S < %s | FileCheck %s
; This test caused a compile-time explosion in ValueTracking
; because we cloned the assume value and call greatly and
; then spent far too long analyzing those.
define void @PR49171(i32 %arg) {
; CHECK-LABEL: @PR49171(
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[ASSUME:%.*]] = icmp eq i32 0, [[ARG:%.*]]
; CHECK-NEXT: call void @llvm.assume(i1 [[ASSUME]])
; CHECK-NEXT: ret void
;
entry:
br label %loop
loop:
%iv = phi i32 [ 0, %entry ], [ %iv.inc, %loop ]
%iv2 = phi i32 [ 0, %entry ], [ %arg, %loop ]
%assume = icmp eq i32 %iv2, %arg
call void @llvm.assume(i1 %assume)
%iv.inc = add nuw nsw i32 %iv, 1
%exitcond = icmp eq i32 %iv.inc, 50 ; Increase me!
br i1 %exitcond, label %exit, label %loop
exit:
ret void
}
declare void @llvm.assume(i1)