[llvm] r307843 - [LoopUnrollRuntime] NFC: Refactored safety checks of unrolling multi-exit loop
Anna Thomas via llvm-commits
llvm-commits at lists.llvm.org
Wed Jul 12 13:55:43 PDT 2017
Author: annat
Date: Wed Jul 12 13:55:43 2017
New Revision: 307843
URL: http://llvm.org/viewvc/llvm-project?rev=307843&view=rev
Log:
[LoopUnrollRuntime] NFC: Refactored safety checks of unrolling multi-exit loop
Refactored the code and separated out a function
`canSafelyUnrollMultiExitLoop` to reduce redundant checks and make it
easier to add profitability heuristics later.
Added tests to runtime unrolling to make sure that unrolling for
multi-exit loops is not done unless the option
-unroll-runtime-multi-exit is true.
Modified:
llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp
llvm/trunk/test/Transforms/LoopUnroll/runtime-loop.ll
Modified: llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp?rev=307843&r1=307842&r2=307843&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/LoopUnrollRuntime.cpp Wed Jul 12 13:55:43 2017
@@ -427,6 +427,50 @@ CloneLoopBlocks(Loop *L, Value *NewIter,
return nullptr;
}
+/// Returns true if we can safely unroll a multi-exit/exiting loop. OtherExits
+/// is populated with all the loop exit blocks other than the LatchExit block.
+static bool
+canSafelyUnrollMultiExitLoop(Loop *L, SmallVectorImpl<BasicBlock *> &OtherExits,
+ BasicBlock *LatchExit, bool PreserveLCSSA,
+ bool UseEpilogRemainder) {
+
+ // Support runtime unrolling for multiple exit blocks and multiple exiting
+ // blocks.
+ if (!UnrollRuntimeMultiExit)
+ return false;
+ // Even if runtime multi exit is enabled, we currently have some correctness
+ // constrains in unrolling a multi-exit loop.
+ // We rely on LCSSA form being preserved when the exit blocks are transformed.
+ if (!PreserveLCSSA)
+ return false;
+ SmallVector<BasicBlock *, 4> Exits;
+ L->getUniqueExitBlocks(Exits);
+ for (auto *BB : Exits)
+ if (BB != LatchExit)
+ OtherExits.push_back(BB);
+
+ // TODO: Support multiple exiting blocks jumping to the `LatchExit` when
+ // UnrollRuntimeMultiExit is true. This will need updating the logic in
+ // connectEpilog/connectProlog.
+ if (!LatchExit->getSinglePredecessor()) {
+ DEBUG(dbgs() << "Bailout for multi-exit handling when latch exit has >1 "
+ "predecessor.\n");
+ return false;
+ }
+ // FIXME: We bail out of multi-exit unrolling when epilog loop is generated
+ // and L is an inner loop. This is because in presence of multiple exits, the
+ // outer loop is incorrect: we do not add the EpilogPreheader and exit to the
+ // outer loop. This is automatically handled in the prolog case, so we do not
+ // have that bug in prolog generation.
+ if (UseEpilogRemainder && L->getParentLoop())
+ return false;
+
+ // All constraints have been satisfied.
+ return true;
+}
+
+
+
/// Insert code in the prolog/epilog code when unrolling a loop with a
/// run-time trip-count.
///
@@ -470,16 +514,8 @@ bool llvm::UnrollRuntimeLoopRemainder(Lo
bool UseEpilogRemainder,
LoopInfo *LI, ScalarEvolution *SE,
DominatorTree *DT, bool PreserveLCSSA) {
- bool hasMultipleExitingBlocks = !L->getExitingBlock();
DEBUG(dbgs() << "Trying runtime unrolling on Loop: \n");
DEBUG(L->dump());
- // Support only single exiting block unless UnrollRuntimeMultiExit is true.
- if (!UnrollRuntimeMultiExit && hasMultipleExitingBlocks) {
- DEBUG(
- dbgs()
- << "Multiple exiting blocks and UnrollRuntimeMultiExit not enabled!\n");
- return false;
- }
// Make sure the loop is in canonical form.
if (!L->isLoopSimplifyForm()) {
@@ -491,52 +527,27 @@ bool llvm::UnrollRuntimeLoopRemainder(Lo
BasicBlock *Latch = L->getLoopLatch();
BasicBlock *Header = L->getHeader();
- BasicBlock *LatchExit = L->getUniqueExitBlock(); // successor out of loop
- if (!LatchExit && !UnrollRuntimeMultiExit) {
- DEBUG(dbgs() << "No unique exit block and UnrollRuntimeMultiExit not enabled\n");
- return false;
- }
- // These are exit blocks other than the target of the latch exiting block.
- SmallVector<BasicBlock *, 4> OtherExits;
BranchInst *LatchBR = cast<BranchInst>(Latch->getTerminator());
- unsigned int ExitIndex = LatchBR->getSuccessor(0) == Header ? 1 : 0;
+ unsigned ExitIndex = LatchBR->getSuccessor(0) == Header ? 1 : 0;
+ BasicBlock *LatchExit = LatchBR->getSuccessor(ExitIndex);
// Cloning the loop basic blocks (`CloneLoopBlocks`) requires that one of the
// targets of the Latch be an exit block out of the loop. This needs
// to be guaranteed by the callers of UnrollRuntimeLoopRemainder.
- assert(!L->contains(LatchBR->getSuccessor(ExitIndex)) &&
+ assert(!L->contains(LatchExit) &&
"one of the loop latch successors should be the exit block!");
- // Support runtime unrolling for multiple exit blocks and multiple exiting
- // blocks.
- if (!LatchExit) {
- LatchExit = LatchBR->getSuccessor(ExitIndex);
- // We rely on LCSSA form being preserved when the exit blocks are
- // transformed.
- if (!PreserveLCSSA)
- return false;
- SmallVector<BasicBlock *, 4> Exits;
- L->getUniqueExitBlocks(Exits);
- for (auto *BB : Exits)
- if (BB != LatchExit)
- OtherExits.push_back(BB);
- }
-
- assert(LatchExit && "Latch Exit should exist!");
-
- // TODO: Support multiple exiting blocks jumping to the `LatchExit` when
- // UnrollRuntimeMultiExit is true. This will need updating the logic in
- // connectEpilog.
- if (!LatchExit->getSinglePredecessor()) {
- DEBUG(dbgs() << "Bailout for multi-exit handling when latch exit has >1 "
- "predecessor.\n");
+ // These are exit blocks other than the target of the latch exiting block.
+ SmallVector<BasicBlock *, 4> OtherExits;
+ bool isMultiExitUnrollingEnabled = canSafelyUnrollMultiExitLoop(
+ L, OtherExits, LatchExit, PreserveLCSSA, UseEpilogRemainder);
+ // Support only single exit and exiting block unless multi-exit loop unrolling is enabled.
+ if (!isMultiExitUnrollingEnabled &&
+ (!L->getExitingBlock() || OtherExits.size())) {
+ DEBUG(
+ dbgs()
+ << "Multiple exit/exiting blocks in loop and multi-exit unrolling not "
+ "enabled!\n");
return false;
}
- // FIXME: We bail out of multi-exit unrolling when epilog loop is generated
- // and L is an inner loop. This is because in presence of multiple exits, the
- // outer loop is incorrect: we do not add the EpilogPreheader and exit to the
- // outer loop. This is automatically handled in the prolog case, so we do not
- // have that bug in prolog generation.
- if (hasMultipleExitingBlocks && UseEpilogRemainder && L->getParentLoop())
- return false;
// Use Scalar Evolution to compute the trip count. This allows more loops to
// be unrolled than relying on induction var simplification.
if (!SE)
Modified: llvm/trunk/test/Transforms/LoopUnroll/runtime-loop.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopUnroll/runtime-loop.ll?rev=307843&r1=307842&r2=307843&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/LoopUnroll/runtime-loop.ll (original)
+++ llvm/trunk/test/Transforms/LoopUnroll/runtime-loop.ll Wed Jul 12 13:55:43 2017
@@ -170,6 +170,74 @@ for.end:
ret i16 %res.0.lcssa
}
+; dont unroll loop with multiple exit/exiting blocks, unless
+; -runtime-unroll-multi-exit=true
+; single exit, multiple exiting blocks.
+define void @unique_exit(i32 %arg) {
+; PROLOG: unique_exit(
+; PROLOG-NOT: .unr
+
+; EPILOG: unique_exit(
+; EPILOG-NOT: .unr
+entry:
+ %tmp = icmp sgt i32 undef, %arg
+ br i1 %tmp, label %preheader, label %returnblock
+
+preheader: ; preds = %entry
+ br label %header
+
+LoopExit: ; preds = %header, %latch
+ %tmp2.ph = phi i32 [ %tmp4, %header ], [ -1, %latch ]
+ br label %returnblock
+
+returnblock: ; preds = %LoopExit, %entry
+ %tmp2 = phi i32 [ -1, %entry ], [ %tmp2.ph, %LoopExit ]
+ ret void
+
+header: ; preds = %preheader, %latch
+ %tmp4 = phi i32 [ %inc, %latch ], [ %arg, %preheader ]
+ %inc = add nsw i32 %tmp4, 1
+ br i1 true, label %LoopExit, label %latch
+
+latch: ; preds = %header
+ %cmp = icmp slt i32 %inc, undef
+ br i1 %cmp, label %header, label %LoopExit
+}
+
+; multiple exit blocks. don't unroll
+define void @multi_exit(i64 %trip, i1 %cond) {
+; PROLOG: multi_exit(
+; PROLOG-NOT: .unr
+
+; EPILOG: multi_exit(
+; EPILOG-NOT: .unr
+entry:
+ br label %loop_header
+
+loop_header:
+ %iv = phi i64 [ 0, %entry ], [ %iv_next, %loop_latch ]
+ br i1 %cond, label %loop_latch, label %loop_exiting_bb1
+
+loop_exiting_bb1:
+ br i1 false, label %loop_exiting_bb2, label %exit1
+
+loop_exiting_bb2:
+ br i1 false, label %loop_latch, label %exit3
+
+exit3:
+ ret void
+
+loop_latch:
+ %iv_next = add i64 %iv, 1
+ %cmp = icmp ne i64 %iv_next, %trip
+ br i1 %cmp, label %loop_header, label %exit2.loopexit
+
+exit1:
+ ret void
+
+exit2.loopexit:
+ ret void
+}
!0 = distinct !{!0, !1}
!1 = !{!"llvm.loop.unroll.runtime.disable"}
More information about the llvm-commits
mailing list