Compute safety information in a much finer granularity.

Summary:
Instead of keeping a variable indicating whether there are early exits
in the loop.  We keep all the early exits. This improves LICM's ability to
move instructions out of the loop based on is-guaranteed-to-execute.

I am going to update compilation time as well soon.

Reviewers: hfinkel, sanjoy, efriedma, mkuper

Reviewed By: hfinkel

Subscribers: llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D32433

llvm-svn: 301196

1 files changed, 8 insertions, 16 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp
index 175d013a..062432f 100644
--- a/llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -1044,21 +1044,11 @@ bool llvm::isGuaranteedToExecute(const Instruction &Inst,
                                  const DominatorTree *DT, const Loop *CurLoop,
                                  const LoopSafetyInfo *SafetyInfo) {
   // We have to check to make sure that the instruction dominates all
-  // of the exit blocks.  If it doesn't, then there is a path out of the loop
-  // which does not execute this instruction, so we can't hoist it.
-
-  // If the instruction is in the header block for the loop (which is very
-  // common), it is always guaranteed to dominate the exit blocks.  Since this
-  // is a common case, and can save some work, check it now.
-  if (Inst.getParent() == CurLoop->getHeader())
-    // If there's a throw in the header block, we can't guarantee we'll reach
-    // Inst.
-    return !SafetyInfo->HeaderMayThrow;
-
-  // Somewhere in this loop there is an instruction which may throw and make us
-  // exit the loop.
-  if (SafetyInfo->MayThrow)
-    return false;
+  // of the exit points.  If it doesn't, then there is a path out of the loop
+  // which does not execute this instruction and its not guaranteed to execute.
+  for (Instruction *ExitInst : SafetyInfo->EarlyExits)
+    if (!DT->dominates(&Inst, ExitInst))
+      return false;
 
   // Get the exit blocks for the current loop.
   SmallVector<BasicBlock *, 8> ExitBlocks;
@@ -1071,7 +1061,9 @@ bool llvm::isGuaranteedToExecute(const Instruction &Inst,
 
   // As a degenerate case, if the loop is statically infinite then we haven't
   // proven anything since there are no exit blocks.
-  if (ExitBlocks.empty())
+  // However, we also special case instruction from the header as the header
+  // is always guaranteed to execute.
+  if (ExitBlocks.empty() && Inst.getParent() != CurLoop->getHeader())
     return false;
 
   // FIXME: In general, we have to prove that the loop isn't an infinite loop.