From f7d5daa0c0b47351a66f7d12f35247636934e7aa Mon Sep 17 00:00:00 2001
From: Geoff Berry <gberry@codeaurora.org>
Date: Fri, 14 Jul 2017 20:13:21 +0000
Subject: [PATCH] [EarlyCSE] Handle calls with no MemorySSA info.

Summary:
When checking for memory dependencies between calls using MemorySSA,
handle cases where the calls have no MemoryAccess associated with them
because the AA analysis being used has determined that the call does not
read/write memory.

Fixes PR33756

Reviewers: dberlin, davide

Subscribers: mcrosier, llvm-commits, Prazek

Differential Revision: https://reviews.llvm.org/D35317

llvm-svn: 308051
---
 llvm/lib/Transforms/Scalar/EarlyCSE.cpp       | 16 +++++++++++-
 .../EarlyCSE/globalsaa-memoryssa.ll           | 25 +++++++++++++++++++
 2 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/Transforms/EarlyCSE/globalsaa-memoryssa.ll

diff --git a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
index 7fd77a082b82..c5c9b2c185d6 100644
--- a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
+++ b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -562,13 +562,27 @@ bool EarlyCSE::isSameMemGeneration(unsigned EarlierGeneration,
   if (!MSSA)
     return false;
 
+  // If MemorySSA has determined that one of EarlierInst or LaterInst does not
+  // read/write memory, then we can safely return true here.
+  // FIXME: We could be more aggressive when checking doesNotAccessMemory(),
+  // onlyReadsMemory(), mayReadFromMemory(), and mayWriteToMemory() in this pass
+  // by also checking the MemorySSA MemoryAccess on the instruction.  Initial
+  // experiments suggest this isn't worthwhile, at least for C/C++ code compiled
+  // with the default optimization pipeline.
+  auto *EarlierMA = MSSA->getMemoryAccess(EarlierInst);
+  if (!EarlierMA)
+    return true;
+  auto *LaterMA = MSSA->getMemoryAccess(LaterInst);
+  if (!LaterMA)
+    return true;
+
   // Since we know LaterDef dominates LaterInst and EarlierInst dominates
   // LaterInst, if LaterDef dominates EarlierInst then it can't occur between
   // EarlierInst and LaterInst and neither can any other write that potentially
   // clobbers LaterInst.
   MemoryAccess *LaterDef =
       MSSA->getWalker()->getClobberingMemoryAccess(LaterInst);
-  return MSSA->dominates(LaterDef, MSSA->getMemoryAccess(EarlierInst));
+  return MSSA->dominates(LaterDef, EarlierMA);
 }
 
 bool EarlyCSE::processNode(DomTreeNode *Node) {
diff --git a/llvm/test/Transforms/EarlyCSE/globalsaa-memoryssa.ll b/llvm/test/Transforms/EarlyCSE/globalsaa-memoryssa.ll
new file mode 100644
index 000000000000..57dbdd883190
--- /dev/null
+++ b/llvm/test/Transforms/EarlyCSE/globalsaa-memoryssa.ll
@@ -0,0 +1,25 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -S -globals-aa -early-cse-memssa | FileCheck %s
+
+define i16 @f1() readonly {
+  ret i16 0
+}
+
+declare void @f2()
+
+; Check that EarlyCSE correctly handles function calls that don't have
+; a MemoryAccess.  In this case the calls to @f1 have no
+; MemoryAccesses since globals-aa determines that @f1 doesn't
+; read/write memory at all.
+
+define void @f3() {
+; CHECK-LABEL: @f3(
+; CHECK-NEXT:    [[CALL1:%.*]] = call i16 @f1()
+; CHECK-NEXT:    call void @f2()
+; CHECK-NEXT:    ret void
+;
+  %call1 = call i16 @f1()
+  call void @f2()
+  %call2 = call i16 @f1()
+  ret void
+}