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[JumpThreading] Restrict PRE across instructions that don't pass control to successors

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PRE in JumpThreading should not be able to hoist copy of non-speculable loads across
instructions that don't always transfer execution to their successors, otherwise they may
introduce an unsafe load which otherwise would not be executed.

The same problem for GVN was fixed as rL316975.

Differential Revision: https://reviews.llvm.org/D40347

llvm-svn: 321063
This commit is contained in:
Max Kazantsev 2017-12-19 09:10:21 +00:00
parent aba02c9095
commit 73d7a6bc49
2 changed files with 117 additions and 0 deletions
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14
lib/Transforms/Scalar/JumpThreading.cpp
View File

@ -1333,6 +1333,20 @@ bool JumpThreadingPass::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
// code size.
BasicBlock *UnavailablePred = nullptr;
// If the value is unavailable in one of predecessors, we will end up
// inserting a new instruction into them. It is only valid if all the
// instructions before LI are guaranteed to pass execution to its successor,
// or if LI is safe to speculate.
// TODO: If this logic becomes more complex, and we will perform PRE insertion
// farther than to a predecessor, we need to reuse the code from GVN's PRE.
// It requires domination tree analysis, so for this simple case it is an
// overkill.
if (PredsScanned.size() != AvailablePreds.size() &&
!isSafeToSpeculativelyExecute(LI))
for (auto I = LoadBB->begin(); &*I != LI; ++I)
if (!isGuaranteedToTransferExecutionToSuccessor(&*I))
return false;
// If there is exactly one predecessor where the value is unavailable, the
// already computed 'OneUnavailablePred' block is it. If it ends in an
// unconditional branch, we know that it isn't a critical edge.

103
test/Transforms/JumpThreading/guards.ll
View File

@ -278,3 +278,106 @@ L2:
L3:
ret void
}
; Make sure that we don't PRE a non-speculable load across a guard.
define void @unsafe_pre_across_guard(i8* %p, i1 %load.is.valid) {
; CHECK-LABEL: @unsafe_pre_across_guard(
; CHECK-NOT: loaded.pr
; CHECK: entry:
; CHECK-NEXT: br label %loop
; CHECK: loop:
; CHECK-NEXT: call void (i1, ...) @llvm.experimental.guard(i1 %load.is.valid) [ "deopt"() ]
; CHECK-NEXT: %loaded = load i8, i8* %p
; CHECK-NEXT: %continue = icmp eq i8 %loaded, 0
; CHECK-NEXT: br i1 %continue, label %exit, label %loop
entry:
br label %loop
loop: ; preds = %loop, %entry
call void (i1, ...) @llvm.experimental.guard(i1 %load.is.valid) [ "deopt"() ]
%loaded = load i8, i8* %p
%continue = icmp eq i8 %loaded, 0
br i1 %continue, label %exit, label %loop
exit: ; preds = %loop
ret void
}
; Make sure that we can safely PRE a speculable load across a guard.
define void @safe_pre_across_guard(i8* noalias nocapture readonly dereferenceable(8) %p, i1 %load.is.valid) {
; CHECK-LABEL: @safe_pre_across_guard(
; CHECK: entry:
; CHECK-NEXT: %loaded.pr = load i8, i8* %p
; CHECK-NEXT: br label %loop
; CHECK: loop:
; CHECK-NEXT: %loaded = phi i8 [ %loaded, %loop ], [ %loaded.pr, %entry ]
; CHECK-NEXT: call void (i1, ...) @llvm.experimental.guard(i1 %load.is.valid) [ "deopt"() ]
; CHECK-NEXT: %continue = icmp eq i8 %loaded, 0
; CHECK-NEXT: br i1 %continue, label %exit, label %loop
entry:
br label %loop
loop: ; preds = %loop, %entry
call void (i1, ...) @llvm.experimental.guard(i1 %load.is.valid) [ "deopt"() ]
%loaded = load i8, i8* %p
%continue = icmp eq i8 %loaded, 0
br i1 %continue, label %exit, label %loop
exit: ; preds = %loop
ret void
}
; Make sure that we don't PRE a non-speculable load across a call which may
; alias with the load.
define void @unsafe_pre_across_call(i8* %p) {
; CHECK-LABEL: @unsafe_pre_across_call(
; CHECK-NOT: loaded.pr
; CHECK: entry:
; CHECK-NEXT: br label %loop
; CHECK: loop:
; CHECK-NEXT: call i32 @f1()
; CHECK-NEXT: %loaded = load i8, i8* %p
; CHECK-NEXT: %continue = icmp eq i8 %loaded, 0
; CHECK-NEXT: br i1 %continue, label %exit, label %loop
entry:
br label %loop
loop: ; preds = %loop, %entry
call i32 @f1()
%loaded = load i8, i8* %p
%continue = icmp eq i8 %loaded, 0
br i1 %continue, label %exit, label %loop
exit: ; preds = %loop
ret void
}
; Make sure that we can safely PRE a speculable load across a call.
define void @safe_pre_across_call(i8* noalias nocapture readonly dereferenceable(8) %p) {
; CHECK-LABEL: @safe_pre_across_call(
; CHECK: entry:
; CHECK-NEXT: %loaded.pr = load i8, i8* %p
; CHECK-NEXT: br label %loop
; CHECK: loop:
; CHECK-NEXT: %loaded = phi i8 [ %loaded, %loop ], [ %loaded.pr, %entry ]
; CHECK-NEXT: call i32 @f1()
; CHECK-NEXT: %continue = icmp eq i8 %loaded, 0
; CHECK-NEXT: br i1 %continue, label %exit, label %loop
entry:
br label %loop
loop: ; preds = %loop, %entry
call i32 @f1()
%loaded = load i8, i8* %p
%continue = icmp eq i8 %loaded, 0
br i1 %continue, label %exit, label %loop
exit: ; preds = %loop
ret void
}