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[Attributor] Deal with shrinking dereferenceability in a loop 

Summary:
If we have a loop in which the dereferenceability of a pointer decreases
we did slowly decrease it iteration by iteration, leading to a timeout.
With this patch we detect such circular reasoning and indicate a
fixpoint early.

Reviewers: uenoku, sstefan1

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66558

llvm-svn: 369784
This commit is contained in:
Johannes Doerfert 2019-08-23 17:29:23 +00:00
parent ccc272cd32
commit 785fad3202
2 changed files with 50 additions and 8 deletions
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23
llvm/lib/Transforms/IPO/Attributor.cpp
View File

@ -1988,16 +1988,27 @@ struct AADereferenceableFloating : AADereferenceableImpl {
// For now we do not try to "increase" dereferenceability due to negative
// indices as we first have to come up with code to deal with loops and
// for overflows of the dereferenceable bytes.
if (Offset.getSExtValue() < 0)
int64_t OffsetSExt = Offset.getSExtValue();
if (OffsetSExt < 0)
Offset = 0;
T.takeAssumedDerefBytesMinimum(
std::max(int64_t(0), DerefBytes - Offset.getSExtValue()));
std::max(int64_t(0), DerefBytes - OffsetSExt));
if (!Stripped && this == &AA) {
T.takeKnownDerefBytesMaximum(
std::max(int64_t(0), DerefBytes - Offset.getSExtValue()));
T.indicatePessimisticFixpoint();
if (this == &AA) {
if (!Stripped) {
// If nothing was stripped IR information is all we got.
T.takeKnownDerefBytesMaximum(
std::max(int64_t(0), DerefBytes - OffsetSExt));
T.indicatePessimisticFixpoint();
} else if (OffsetSExt > 0) {
// If something was stripped but there is circular reasoning we look
// for the offset. If it is positive we basically decrease the
// dereferenceable bytes in a circluar loop now, which will simply
// drive them down to the known value in a very slow way which we
// can accelerate.
T.indicatePessimisticFixpoint();
}
}
return T.isValidState();

35
llvm/test/Transforms/FunctionAttrs/dereferenceable.ll
View File

@ -1,6 +1,8 @@
; RUN: opt -attributor --attributor-disable=false -attributor-max-iterations=4 -S < %s | FileCheck %s --check-prefixes=ATTRIBUTOR
declare void @deref_phi_user(i32* %a);
; TEST 1
; take mininimum of return values
;
@ -52,8 +54,7 @@ define dereferenceable(4) i32* @test4(i32* dereferenceable(8) %0) local_unnamed_
; TEST 5
; loop in which dereferenceabily "grows"
declare void @deref_phi_user(i32* %a);
define void @deref_phi(i32* dereferenceable(4000) %a) {
define void @deref_phi_growing(i32* dereferenceable(4000) %a) {
entry:
br label %for.cond
@ -80,3 +81,33 @@ for.inc: ; preds = %for.body
for.end: ; preds = %for.cond.cleanup
ret void
}
; TEST 6
; loop in which dereferenceabily "shrinks"
define void @deref_phi_shrinking(i32* dereferenceable(4000) %a) {
entry:
br label %for.cond
for.cond: ; preds = %for.inc, %entry
%i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
%a.addr.0 = phi i32* [ %a, %entry ], [ %incdec.ptr, %for.inc ]
; CHECK: call void @deref_phi_user(i32* %a.addr.0)
call void @deref_phi_user(i32* %a.addr.0)
%tmp = load i32, i32* %a.addr.0, align 4
%cmp = icmp slt i32 %i.0, %tmp
br i1 %cmp, label %for.body, label %for.cond.cleanup
for.cond.cleanup: ; preds = %for.cond
br label %for.end
for.body: ; preds = %for.cond
br label %for.inc
for.inc: ; preds = %for.body
%incdec.ptr = getelementptr inbounds i32, i32* %a.addr.0, i64 1
%inc = add nuw nsw i32 %i.0, 1
br label %for.cond
for.end: ; preds = %for.cond.cleanup
ret void
}