[LICM] Make isGuaranteedToExecute more accurate.

Summary:
Make isGuaranteedToExecute use the
isGuaranteedToTransferExecutionToSuccessor helper, and make that helper
a bit more accurate.

There's a potential performance impact here from assuming that arbitrary
calls might not return. This probably has little impact on loads and
stores to a pointer because most things alias analysis can reason about
are dereferenceable anyway. The other impacts, like less aggressive
hoisting of sdiv by a variable and less aggressive hoisting around
volatile memory operations, are unlikely to matter for real code.

This also impacts SCEV, which uses the same helper.  It's a minor
improvement there because we can tell that, for example, memcpy always
returns normally. Strictly speaking, it's also introducing
a bug, but it's not any worse than everywhere else we assume readonly
functions terminate.

Fixes http://llvm.org/PR27857.

Reviewers: hfinkel, reames, chandlerc, sanjoy

Subscribers: broune, llvm-commits

Differential Revision: http://reviews.llvm.org/D21167

llvm-svn: 272489

2 files changed, 5 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index 8f7cacedfb4..b67bd24ecb8 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -388,7 +388,7 @@ void llvm::computeLoopSafetyInfo(LoopSafetyInfo *SafetyInfo, Loop *CurLoop) {
   // Iterate over header and compute safety info.
   for (BasicBlock::iterator I = Header->begin(), E = Header->end();
        (I != E) && !SafetyInfo->HeaderMayThrow; ++I)
-    SafetyInfo->HeaderMayThrow |= I->mayThrow();
+    SafetyInfo->HeaderMayThrow |= !isGuaranteedToTransferExecutionToSuccessor(&*I);
 
   SafetyInfo->MayThrow = SafetyInfo->HeaderMayThrow;
   // Iterate over loop instructions and compute safety info.
@@ -397,7 +397,7 @@ void llvm::computeLoopSafetyInfo(LoopSafetyInfo *SafetyInfo, Loop *CurLoop) {
        (BB != BBE) && !SafetyInfo->MayThrow; ++BB)
     for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end();
          (I != E) && !SafetyInfo->MayThrow; ++I)
-      SafetyInfo->MayThrow |= I->mayThrow();
+      SafetyInfo->MayThrow |= !isGuaranteedToTransferExecutionToSuccessor(&*I);
 
   // Compute funclet colors if we might sink/hoist in a function with a funclet
   // personality routine.
diff --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp
index 300931352db..3902c67c6a0 100644
--- a/llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -962,5 +962,8 @@ bool llvm::isGuaranteedToExecute(const Instruction &Inst,
   if (ExitBlocks.empty())
     return false;
 
+  // FIXME: In general, we have to prove that the loop isn't an infinite loop.
+  // See http::llvm.org/PR24078 .  (The "ExitBlocks.empty()" check above is
+  // just a special case of this.)
   return true;
 }