Add an AlignmentFromAssumptions Pass

This adds a ScalarEvolution-powered transformation that updates load, store and
memory intrinsic pointer alignments based on invariant((a+q) & b == 0)
expressions. Many of the simple cases we can get with ValueTracking, but we
still need something like this for the more complicated cases (such as those
with an offset) that require some algebra. Note that gcc's
__builtin_assume_aligned's optional third argument provides exactly for this
kind of 'misalignment' offset for which this kind of logic is necessary.

The primary motivation is to fixup alignments for vector loads/stores after
vectorization (and unrolling). This pass is added to the optimization pipeline
just after the SLP vectorizer runs (which, admittedly, does not preserve SE,
although I imagine it could).  Regardless, I actually don't think that the
preservation matters too much in this case: SE computes lazily, and this pass
won't issue any SE queries unless there are any assume intrinsics, so there
should be no real additional cost in the common case (SLP does preserve DT and
LoopInfo).

llvm-svn: 217344

4 files changed, 12 insertions, 0 deletions

diff --git a/llvm/include/llvm-c/Transforms/Scalar.h b/llvm/include/llvm-c/Transforms/Scalar.h
index c0d670e3050..a8ad772624b 100644
--- a/llvm/include/llvm-c/Transforms/Scalar.h
+++ b/llvm/include/llvm-c/Transforms/Scalar.h
@@ -35,6 +35,9 @@ extern "C" {
 /** See llvm::createAggressiveDCEPass function. */
 void LLVMAddAggressiveDCEPass(LLVMPassManagerRef PM);
 
+/** See llvm::createAlignmentFromAssumptionsPass function. */
+void LLVMAddAlignmentFromAssumptionsPass(LLVMPassManagerRef PM);
+
 /** See llvm::createCFGSimplificationPass function. */
 void LLVMAddCFGSimplificationPass(LLVMPassManagerRef PM);
 
diff --git a/llvm/include/llvm/InitializePasses.h b/llvm/include/llvm/InitializePasses.h
index b1dea4fa19f..b0ec376267e 100644
--- a/llvm/include/llvm/InitializePasses.h
+++ b/llvm/include/llvm/InitializePasses.h
@@ -73,6 +73,7 @@ void initializeAlwaysInlinerPass(PassRegistry&);
 void initializeArgPromotionPass(PassRegistry&);
 void initializeAtomicExpandPass(PassRegistry&);
 void initializeSampleProfileLoaderPass(PassRegistry&);
+void initializeAlignmentFromAssumptionsPass(PassRegistry&);
 void initializeBarrierNoopPass(PassRegistry&);
 void initializeBasicAliasAnalysisPass(PassRegistry&);
 void initializeCallGraphWrapperPassPass(PassRegistry &);
diff --git a/llvm/include/llvm/LinkAllPasses.h b/llvm/include/llvm/LinkAllPasses.h
index 12ae11e4ee5..81e82638255 100644
--- a/llvm/include/llvm/LinkAllPasses.h
+++ b/llvm/include/llvm/LinkAllPasses.h
@@ -52,6 +52,7 @@ namespace {
       (void) llvm::createAliasAnalysisCounterPass();
       (void) llvm::createAliasDebugger();
       (void) llvm::createArgumentPromotionPass();
+      (void) llvm::createAlignmentFromAssumptionsPass();
       (void) llvm::createBasicAliasAnalysisPass();
       (void) llvm::createLibCallAliasAnalysisPass(nullptr);
       (void) llvm::createScalarEvolutionAliasAnalysisPass();
diff --git a/llvm/include/llvm/Transforms/Scalar.h b/llvm/include/llvm/Transforms/Scalar.h
index 413134e51c7..7a561ee5b24 100644
--- a/llvm/include/llvm/Transforms/Scalar.h
+++ b/llvm/include/llvm/Transforms/Scalar.h
@@ -36,6 +36,13 @@ FunctionPass *createConstantPropagationPass();
 
 //===----------------------------------------------------------------------===//
 //
+// AlignmentFromAssumptions - Use assume intrinsics to set load/store
+// alignments.
+//
+FunctionPass *createAlignmentFromAssumptionsPass();
+
+//===----------------------------------------------------------------------===//
+//
 // SCCP - Sparse conditional constant propagation.
 //
 FunctionPass *createSCCPPass();