[llvm] r321063 - [JumpThreading] Restrict PRE across instructions that don't pass control to successors
Max Kazantsev via llvm-commits
llvm-commits at lists.llvm.org
Tue Dec 19 01:10:21 PST 2017
Author: mkazantsev
Date: Tue Dec 19 01:10:21 2017
New Revision: 321063
URL: http://llvm.org/viewvc/llvm-project?rev=321063&view=rev
Log:
[JumpThreading] Restrict PRE across instructions that don't pass control to successors
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
Modified:
llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
llvm/trunk/test/Transforms/JumpThreading/guards.ll
Modified: llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp?rev=321063&r1=321062&r2=321063&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp Tue Dec 19 01:10:21 2017
@@ -1333,6 +1333,20 @@ bool JumpThreadingPass::SimplifyPartiall
// 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.
Modified: llvm/trunk/test/Transforms/JumpThreading/guards.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/JumpThreading/guards.ll?rev=321063&r1=321062&r2=321063&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/JumpThreading/guards.ll (original)
+++ llvm/trunk/test/Transforms/JumpThreading/guards.ll Tue Dec 19 01:10:21 2017
@@ -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
+}
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