[clang] 6f1503d - [LoopDeletion] Allows deletion of possibly infinite side-effect free loops
via cfe-commits
cfe-commits at lists.llvm.org
Wed Dec 30 18:43:21 PST 2020
Author: Atmn Patel
Date: 2020-12-30T21:43:01-05:00
New Revision: 6f1503d59854b331f1f970d39839619b0a26bbc7
URL: https://github.com/llvm/llvm-project/commit/6f1503d59854b331f1f970d39839619b0a26bbc7
DIFF: https://github.com/llvm/llvm-project/commit/6f1503d59854b331f1f970d39839619b0a26bbc7.diff
LOG: [LoopDeletion] Allows deletion of possibly infinite side-effect free loops
>From C11 and C++11 onwards, a forward-progress requirement has been
introduced for both languages. In the case of C, loops with non-constant
conditionals that do not have any observable side-effects (as defined by
6.8.5p6) can be assumed by the implementation to terminate, and in the
case of C++, this assumption extends to all functions. The clang
frontend will emit the `mustprogress` function attribute for C++
functions (D86233, D85393, D86841) and emit the loop metadata
`llvm.loop.mustprogress` for every loop in C11 or later that has a
non-constant conditional.
This patch modifies LoopDeletion so that only loops with
the `llvm.loop.mustprogress` metadata or loops contained in functions
that are required to make progress (`mustprogress` or `willreturn`) are
checked for observable side-effects. If these loops do not have an
observable side-effect, then we delete them.
Loops without observable side-effects that do not satisfy the above
conditions will not be deleted.
Reviewed By: jdoerfert
Differential Revision: https://reviews.llvm.org/D86844
Added:
llvm/test/Transforms/LoopDeletion/mustprogress.ll
Modified:
clang/test/Misc/loop-opt-setup.c
llvm/include/llvm/Transforms/Utils/LoopUtils.h
llvm/lib/Transforms/Scalar/LoopDeletion.cpp
llvm/lib/Transforms/Utils/LoopUtils.cpp
llvm/test/Transforms/LoopDeletion/no-exit-blocks.ll
Removed:
################################################################################
diff --git a/clang/test/Misc/loop-opt-setup.c b/clang/test/Misc/loop-opt-setup.c
index c5c2ec4fda84..660eea25c6af 100644
--- a/clang/test/Misc/loop-opt-setup.c
+++ b/clang/test/Misc/loop-opt-setup.c
@@ -1,4 +1,5 @@
// RUN: %clang -O1 -fno-unroll-loops -S -o - %s -emit-llvm | FileCheck %s
+// RUN: %clang -std=c99 -O1 -fno-unroll-loops -S -o - %s -emit-llvm | FileCheck %s --check-prefix C99
extern int a[16];
int b = 0;
@@ -24,7 +25,12 @@ void Helper() {
}
// Check br i1 to make sure the loop is gone, there will still be a label branch for the infinite loop.
+// In C99, there was no forward progress requirement, so we expect the infinite loop to still exist,
+// but for C11 and onwards, the infinite loop can be deleted.
// CHECK-LABEL: Helper
-// CHECK: br label
-// CHECK-NOT: br i1
-// CHECK: br label
+// C99: br label
+// C99-NOT: br i1
+// C99: br label
+// CHECK: entry:
+// CHECK-NOT: br i1
+// CHECK-NEXT: ret void
diff --git a/llvm/include/llvm/Transforms/Utils/LoopUtils.h b/llvm/include/llvm/Transforms/Utils/LoopUtils.h
index ba2bb0a4c6b0..fc6b72eb28af 100644
--- a/llvm/include/llvm/Transforms/Utils/LoopUtils.h
+++ b/llvm/include/llvm/Transforms/Utils/LoopUtils.h
@@ -255,6 +255,9 @@ bool hasDisableAllTransformsHint(const Loop *L);
/// Look for the loop attribute that disables the LICM transformation heuristics.
bool hasDisableLICMTransformsHint(const Loop *L);
+/// Look for the loop attribute that requires progress within the loop.
+bool hasMustProgress(const Loop *L);
+
/// The mode sets how eager a transformation should be applied.
enum TransformationMode {
/// The pass can use heuristics to determine whether a transformation should
diff --git a/llvm/lib/Transforms/Scalar/LoopDeletion.cpp b/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
index 065db647561e..814cfc7ac6a9 100644
--- a/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
@@ -128,10 +128,11 @@ static bool isLoopNeverExecuted(Loop *L) {
/// Remove a loop if it is dead.
///
-/// A loop is considered dead if it does not impact the observable behavior of
-/// the program other than finite running time. This never removes a loop that
-/// might be infinite (unless it is never executed), as doing so could change
-/// the halting/non-halting nature of a program.
+/// A loop is considered dead either if it does not impact the observable
+/// behavior of the program other than finite running time, or if it is
+/// required to make progress by an attribute such as 'mustprogress' or
+/// 'llvm.loop.mustprogress' and does not make any. This may remove
+/// infinite loops that have been required to make progress.
///
/// This entire process relies pretty heavily on LoopSimplify form and LCSSA in
/// order to make various safety checks work.
@@ -207,11 +208,13 @@ static LoopDeletionResult deleteLoopIfDead(Loop *L, DominatorTree &DT,
: LoopDeletionResult::Unmodified;
}
- // Don't remove loops for which we can't solve the trip count.
- // They could be infinite, in which case we'd be changing program behavior.
+ // Don't remove loops for which we can't solve the trip count unless the loop
+ // was required to make progress but has been determined to be dead.
const SCEV *S = SE.getConstantMaxBackedgeTakenCount(L);
- if (isa<SCEVCouldNotCompute>(S)) {
- LLVM_DEBUG(dbgs() << "Could not compute SCEV MaxBackedgeTakenCount.\n");
+ if (isa<SCEVCouldNotCompute>(S) &&
+ !L->getHeader()->getParent()->mustProgress() && !hasMustProgress(L)) {
+ LLVM_DEBUG(dbgs() << "Could not compute SCEV MaxBackedgeTakenCount and was "
+ "not required to make progress.\n");
return Changed ? LoopDeletionResult::Modified
: LoopDeletionResult::Unmodified;
}
diff --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp
index 8dc7709c6e55..4cd59af6e551 100644
--- a/llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -63,6 +63,7 @@ static cl::opt<bool> ForceReductionIntrinsic(
static const char *LLVMLoopDisableNonforced = "llvm.loop.disable_nonforced";
static const char *LLVMLoopDisableLICM = "llvm.licm.disable";
+static const char *LLVMLoopMustProgress = "llvm.loop.mustprogress";
bool llvm::formDedicatedExitBlocks(Loop *L, DominatorTree *DT, LoopInfo *LI,
MemorySSAUpdater *MSSAU,
@@ -419,6 +420,10 @@ bool llvm::hasDisableLICMTransformsHint(const Loop *L) {
return getBooleanLoopAttribute(L, LLVMLoopDisableLICM);
}
+bool llvm::hasMustProgress(const Loop *L) {
+ return getBooleanLoopAttribute(L, LLVMLoopMustProgress);
+}
+
TransformationMode llvm::hasUnrollTransformation(Loop *L) {
if (getBooleanLoopAttribute(L, "llvm.loop.unroll.disable"))
return TM_SuppressedByUser;
@@ -589,6 +594,7 @@ void llvm::deleteDeadLoop(Loop *L, DominatorTree *DT, ScalarEvolution *SE,
IRBuilder<> Builder(OldBr);
auto *ExitBlock = L->getUniqueExitBlock();
+ DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
if (ExitBlock) {
assert(ExitBlock && "Should have a unique exit block!");
assert(L->hasDedicatedExits() && "Loop should have dedicated exits!");
@@ -620,7 +626,6 @@ void llvm::deleteDeadLoop(Loop *L, DominatorTree *DT, ScalarEvolution *SE,
"Should have exactly one value and that's from the preheader!");
}
- DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
if (DT) {
DTU.applyUpdates({{DominatorTree::Insert, Preheader, ExitBlock}});
if (MSSA) {
@@ -636,27 +641,28 @@ void llvm::deleteDeadLoop(Loop *L, DominatorTree *DT, ScalarEvolution *SE,
Builder.CreateBr(ExitBlock);
// Remove the old branch.
Preheader->getTerminator()->eraseFromParent();
-
- if (DT) {
- DTU.applyUpdates({{DominatorTree::Delete, Preheader, L->getHeader()}});
- if (MSSA) {
- MSSAU->applyUpdates(
- {{DominatorTree::Delete, Preheader, L->getHeader()}}, *DT);
- SmallSetVector<BasicBlock *, 8> DeadBlockSet(L->block_begin(),
- L->block_end());
- MSSAU->removeBlocks(DeadBlockSet);
- if (VerifyMemorySSA)
- MSSA->verifyMemorySSA();
- }
- }
} else {
assert(L->hasNoExitBlocks() &&
"Loop should have either zero or one exit blocks.");
+
Builder.SetInsertPoint(OldBr);
Builder.CreateUnreachable();
Preheader->getTerminator()->eraseFromParent();
}
+ if (DT) {
+ DTU.applyUpdates({{DominatorTree::Delete, Preheader, L->getHeader()}});
+ if (MSSA) {
+ MSSAU->applyUpdates({{DominatorTree::Delete, Preheader, L->getHeader()}},
+ *DT);
+ SmallSetVector<BasicBlock *, 8> DeadBlockSet(L->block_begin(),
+ L->block_end());
+ MSSAU->removeBlocks(DeadBlockSet);
+ if (VerifyMemorySSA)
+ MSSA->verifyMemorySSA();
+ }
+ }
+
// Use a map to unique and a vector to guarantee deterministic ordering.
llvm::SmallDenseSet<std::pair<DIVariable *, DIExpression *>, 4> DeadDebugSet;
llvm::SmallVector<DbgVariableIntrinsic *, 4> DeadDebugInst;
diff --git a/llvm/test/Transforms/LoopDeletion/mustprogress.ll b/llvm/test/Transforms/LoopDeletion/mustprogress.ll
new file mode 100644
index 000000000000..1931040bbaca
--- /dev/null
+++ b/llvm/test/Transforms/LoopDeletion/mustprogress.ll
@@ -0,0 +1,237 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --function-signature --check-attributes
+; RUN: opt < %s -loop-deletion -verify-dom-info -S | FileCheck %s
+
+;; Original C Code:
+;; void unknown_tripcount_mustprogress_attr_mustprogress_loopmd(int a, int b) {
+;; for (; a < b;) ;
+;; for (;;) ;
+;; }
+
+define void @unknown_tripcount_mustprogress_attr_mustprogress_loopmd(i32 %a, i32 %b) #0 {
+; CHECK: Function Attrs: mustprogress
+; CHECK-LABEL: define {{[^@]+}}@unknown_tripcount_mustprogress_attr_mustprogress_loopmd
+; CHECK-SAME: (i32 [[A:%.*]], i32 [[B:%.*]]) [[ATTR0:#.*]] {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br label [[FOR_END:%.*]]
+; CHECK: for.end:
+; CHECK-NEXT: unreachable
+;
+entry:
+ br label %for.cond
+for.cond:
+ %cmp = icmp slt i32 %a, %b
+ br i1 %cmp, label %for.body, label %for.end
+for.body:
+ br label %for.cond, !llvm.loop !2
+for.end:
+ br label %for.cond1
+for.cond1:
+ br label %for.cond1
+}
+
+;; Original C Code:
+;; void unknown_tripcount_mustprogress_attr_no_mustprogress_loopmd(int a, int b) {
+;; for (; a < b;) ;
+;; for (;;) ;
+;; }
+;; => Removed mustprogress loop attribute
+
+define void @unknown_tripcount_mustprogress_attr_no_mustprogess_loopmd(i32 %a, i32 %b) #0 {
+; CHECK: Function Attrs: mustprogress
+; CHECK-LABEL: define {{[^@]+}}@unknown_tripcount_mustprogress_attr_no_mustprogess_loopmd
+; CHECK-SAME: (i32 [[A:%.*]], i32 [[B:%.*]]) [[ATTR0]] {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br label [[FOR_END:%.*]]
+; CHECK: for.end:
+; CHECK-NEXT: unreachable
+;
+entry:
+ br label %for.cond
+for.cond:
+ %cmp = icmp slt i32 %a, %b
+ br i1 %cmp, label %for.body, label %for.end
+for.body:
+ br label %for.cond
+for.end:
+ br label %for.cond1
+for.cond1:
+ br label %for.cond1
+}
+
+;; Original C Code:
+;; void known_tripcount_no_mustprogress_attr_no_mustprogress_loopmd() {
+;; for (int i = 0; i < 5; i++) ;
+;; }
+
+define void @known_tripcount_no_mustprogress_attr_no_mustprogress_loopmd() {
+; CHECK-LABEL: define {{[^@]+}}@known_tripcount_no_mustprogress_attr_no_mustprogress_loopmd() {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br label [[FOR_END:%.*]]
+; CHECK: for.end:
+; CHECK-NEXT: ret void
+;
+entry:
+ br label %for.cond
+for.cond:
+ %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+ %cmp = icmp slt i32 %i.0, 5
+ br i1 %cmp, label %for.body, label %for.end
+for.body:
+ br label %for.inc
+for.inc:
+ %inc = add nsw i32 %i.0, 1
+ br label %for.cond
+for.end:
+ ret void
+}
+
+;; Original C Code:
+;; void known_tripcount_no_mustprogress_attr_mustprogress_loopmd() {
+;; for (int i = 0; i < 5; i++) ;
+;; }
+;; => Added mustprogress loop attribute
+
+define void @known_tripcount_no_mustprogress_attr_mustprogress_loopmd() {
+; CHECK-LABEL: define {{[^@]+}}@known_tripcount_no_mustprogress_attr_mustprogress_loopmd() {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br label [[FOR_END:%.*]]
+; CHECK: for.end:
+; CHECK-NEXT: ret void
+;
+entry:
+ br label %for.cond
+for.cond:
+ %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+ %cmp = icmp slt i32 %i.0, 5
+ br i1 %cmp, label %for.body, label %for.end
+for.body:
+ br label %for.inc
+for.inc:
+ %inc = add nsw i32 %i.0, 1
+ br label %for.cond, !llvm.loop !4
+for.end:
+ ret void
+}
+
+;; Original C Code:
+;; void known_tripcount_mustprogress_attr_no_mustprogress_loopmd() {
+;; for (int i = 0; i < 5; i++) ;
+;; }
+;; => Added mustprogress function attribute
+
+define void @known_tripcount_mustprogress_attr_no_mustprogress_loopmd() #0 {
+; CHECK: Function Attrs: mustprogress
+; CHECK-LABEL: define {{[^@]+}}@known_tripcount_mustprogress_attr_no_mustprogress_loopmd
+; CHECK-SAME: () [[ATTR0]] {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br label [[FOR_END:%.*]]
+; CHECK: for.end:
+; CHECK-NEXT: ret void
+;
+entry:
+ br label %for.cond
+for.cond:
+ %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+ %cmp = icmp slt i32 %i.0, 5
+ br i1 %cmp, label %for.body, label %for.end
+for.body:
+ br label %for.inc
+for.inc:
+ %inc = add nsw i32 %i.0, 1
+ br label %for.cond
+for.end:
+ ret void
+}
+
+;; Original C Code:
+;; void known_tripcount_mustprogress_attr_mustprogress_loopmd() {
+;; for (int i = 0; i < 5; i++) ;
+;; }
+;; => Added mustprogress function and mustprogress loop attribute
+
+define void @known_tripcount_mustprogress_attr_mustprogress_loopmd() #0 {
+; CHECK: Function Attrs: mustprogress
+; CHECK-LABEL: define {{[^@]+}}@known_tripcount_mustprogress_attr_mustprogress_loopmd
+; CHECK-SAME: () [[ATTR0]] {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br label [[FOR_END:%.*]]
+; CHECK: for.end:
+; CHECK-NEXT: ret void
+;
+entry:
+ br label %for.cond
+for.cond:
+ %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+ %cmp = icmp slt i32 %i.0, 5
+ br i1 %cmp, label %for.body, label %for.end
+for.body:
+ br label %for.inc
+for.inc:
+ %inc = add nsw i32 %i.0, 1
+ br label %for.cond, !llvm.loop !5
+for.end:
+ ret void
+}
+
+;; Original C Code:
+;; void unknown_tripcount_no_mustprogress_attr_mustprogress_loopmd(int a, int b) {
+;; for (; a < b;) ;
+;; }
+;; => Added mustprogress loop attribute
+
+define void @unknown_tripcount_no_mustprogress_attr_mustprogress_loopmd(i32 %a, i32 %b) {
+; CHECK-LABEL: define {{[^@]+}}@unknown_tripcount_no_mustprogress_attr_mustprogress_loopmd
+; CHECK-SAME: (i32 [[A:%.*]], i32 [[B:%.*]]) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br label [[FOR_END:%.*]]
+; CHECK: for.end:
+; CHECK-NEXT: ret void
+;
+entry:
+ br label %for.cond
+for.cond:
+ %cmp = icmp slt i32 %a, %b
+ br i1 %cmp, label %for.body, label %for.end
+for.body:
+ br label %for.cond, !llvm.loop !6
+for.end:
+ ret void
+}
+
+;; Original C Code:
+;; void unknown_tripcount_no_mustprogress_attr_no_mustprogress_loopmd(int a, int b) {
+;; for (; a < b;) ;
+;; }
+
+define void @unknown_tripcount_no_mustprogress_attr_no_mustprogress_loopmd(i32 %a, i32 %b) {
+; CHECK-LABEL: define {{[^@]+}}@unknown_tripcount_no_mustprogress_attr_no_mustprogress_loopmd
+; CHECK-SAME: (i32 [[A:%.*]], i32 [[B:%.*]]) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br label [[FOR_COND:%.*]]
+; CHECK: for.cond:
+; CHECK-NEXT: [[CMP:%.*]] = icmp slt i32 [[A]], [[B]]
+; CHECK-NEXT: br i1 [[CMP]], label [[FOR_BODY:%.*]], label [[FOR_END:%.*]]
+; CHECK: for.body:
+; CHECK-NEXT: br label [[FOR_COND]]
+; CHECK: for.end:
+; CHECK-NEXT: ret void
+;
+entry:
+ br label %for.cond
+for.cond:
+ %cmp = icmp slt i32 %a, %b
+ br i1 %cmp, label %for.body, label %for.end
+for.body:
+ br label %for.cond
+for.end:
+ ret void
+}
+
+; CHECK: attributes [[ATTR0]] = { mustprogress }
+
+attributes #0 = { mustprogress }
+!2 = distinct !{!2, !3}
+!3 = !{!"llvm.loop.mustprogress"}
+!4 = distinct !{!4, !3}
+!5 = distinct !{!5, !3}
+!6 = distinct !{!6, !3}
diff --git a/llvm/test/Transforms/LoopDeletion/no-exit-blocks.ll b/llvm/test/Transforms/LoopDeletion/no-exit-blocks.ll
index cd790903e8a1..1e68d4f95301 100644
--- a/llvm/test/Transforms/LoopDeletion/no-exit-blocks.ll
+++ b/llvm/test/Transforms/LoopDeletion/no-exit-blocks.ll
@@ -5,14 +5,7 @@ define void @f() #0 {
; CHECK: Function Attrs: mustprogress
; CHECK-LABEL: define {{[^@]+}}@f
; CHECK-SAME: () [[ATTR0:#.*]] {
-; CHECK-NEXT: br label [[TMP1:%.*]]
-; CHECK: 1:
-; CHECK-NEXT: [[DOT01:%.*]] = phi i32 [ 1, [[TMP0:%.*]] ], [ [[TMP3:%.*]], [[TMP2:%.*]] ]
-; CHECK-NEXT: [[DOT0:%.*]] = phi i32 [ 1, [[TMP0]] ], [ [[TMP3]], [[TMP2]] ]
-; CHECK-NEXT: br label [[TMP2]]
-; CHECK: 2:
-; CHECK-NEXT: [[TMP3]] = add nsw i32 [[DOT01]], [[DOT0]]
-; CHECK-NEXT: br label [[TMP1]]
+; CHECK-NEXT: unreachable
;
br label %1
More information about the cfe-commits
mailing list