diff options
| -rw-r--r-- | llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp | 13 | ||||
| -rw-r--r-- | llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch.ll | 30 |
2 files changed, 43 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp index bbe17ed627f..ff8e1c1bd3c 100644 --- a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp +++ b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp @@ -17,9 +17,11 @@ #include "llvm/ADT/Statistic.h" #include "llvm/ADT/Twine.h" #include "llvm/Analysis/AssumptionCache.h" +#include "llvm/Analysis/CFG.h" #include "llvm/Analysis/CodeMetrics.h" #include "llvm/Analysis/LoopAnalysisManager.h" #include "llvm/Analysis/LoopInfo.h" +#include "llvm/Analysis/LoopIterator.h" #include "llvm/Analysis/LoopPass.h" #include "llvm/IR/BasicBlock.h" #include "llvm/IR/Constant.h" @@ -1938,6 +1940,17 @@ unswitchLoop(Loop &L, DominatorTree &DT, LoopInfo &LI, AssumptionCache &AC, if (UnswitchCandidates.empty()) return Changed; + // Check if there are irreducible CFG cycles in this loop. If so, we cannot + // easily unswitch non-trivial edges out of the loop. Doing so might turn the + // irreducible control flow into reducible control flow and introduce new + // loops "out of thin air". If we ever discover important use cases for doing + // this, we can add support to loop unswitch, but it is a lot of complexity + // for what seems little or no real world benifit. + LoopBlocksRPO RPOT(&L); + RPOT.perform(&LI); + if (containsIrreducibleCFG<const BasicBlock *>(RPOT, LI)) + return Changed; + DEBUG(dbgs() << "Considering " << UnswitchCandidates.size() << " non-trivial loop invariant conditions for unswitching.\n"); diff --git a/llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch.ll b/llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch.ll index 51cdebe70e0..651ba19c427 100644 --- a/llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch.ll +++ b/llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch.ll @@ -2350,3 +2350,33 @@ loop_exit: ; CHECK-NEXT: %[[LCSSA:.*]] = phi i32 [ %[[V]], %loop_begin ] ; CHECK-NEXT: ret i32 %[[LCSSA]] } + +; Negative test: we do not switch when the loop contains unstructured control +; flows as it would significantly complicate the process as novel loops might +; be formed, etc. +define void @test_no_unswitch_unstructured_cfg(i1* %ptr, i1 %cond) { +; CHECK-LABEL: @test_no_unswitch_unstructured_cfg( +entry: + br label %loop_begin + +loop_begin: + br i1 %cond, label %loop_left, label %loop_right + +loop_left: + %v1 = load i1, i1* %ptr + br i1 %v1, label %loop_right, label %loop_merge + +loop_right: + %v2 = load i1, i1* %ptr + br i1 %v2, label %loop_left, label %loop_merge + +loop_merge: + %v3 = load i1, i1* %ptr + br i1 %v3, label %loop_latch, label %loop_exit + +loop_latch: + br label %loop_begin + +loop_exit: + ret void +} |
