From 37ade2b8015d86e73c62ab48a2ce5f0ce10de708 Mon Sep 17 00:00:00 2001
From: Nick Lewycky <nicholas@mxc.ca>
Date: Sun, 7 Jul 2013 10:15:16 +0000
Subject: [PATCH] Eliminate trivial redundant loads across nocapture+readonly
 calls to uncaptured pointer arguments.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185776 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/AliasAnalysis.cpp   | 15 +++++++++++----
 test/Transforms/GVN/readattrs.ll | 17 +++++++++++++++++
 2 files changed, 28 insertions(+), 4 deletions(-)
 create mode 100644 test/Transforms/GVN/readattrs.ll

diff --git a/lib/Analysis/AliasAnalysis.cpp b/lib/Analysis/AliasAnalysis.cpp
index 3454ce0be2b..054930c0e19 100644
--- a/lib/Analysis/AliasAnalysis.cpp
+++ b/lib/Analysis/AliasAnalysis.cpp
@@ -450,6 +450,7 @@ AliasAnalysis::callCapturesBefore(const Instruction *I,
     return AliasAnalysis::ModRef;
 
   unsigned ArgNo = 0;
+  AliasAnalysis::ModRefResult R = AliasAnalysis::NoModRef;
   for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
        CI != CE; ++CI, ++ArgNo) {
     // Only look at the no-capture or byval pointer arguments.  If this
@@ -463,12 +464,18 @@ AliasAnalysis::callCapturesBefore(const Instruction *I,
     // is impossible to alias the pointer we're checking.  If not, we have to
     // assume that the call could touch the pointer, even though it doesn't
     // escape.
-    if (!isNoAlias(AliasAnalysis::Location(*CI),
-                   AliasAnalysis::Location(Object))) {
-      return AliasAnalysis::ModRef;
+    if (isNoAlias(AliasAnalysis::Location(*CI),
+		  AliasAnalysis::Location(Object)))
+      continue;
+    if (CS.doesNotAccessMemory(ArgNo))
+      continue;
+    if (CS.onlyReadsMemory(ArgNo)) {
+      R = AliasAnalysis::Ref;
+      continue;
     }
+    return AliasAnalysis::ModRef;
   }
-  return AliasAnalysis::NoModRef;
+  return R;
 }
 
 // AliasAnalysis destructor: DO NOT move this to the header file for
diff --git a/test/Transforms/GVN/readattrs.ll b/test/Transforms/GVN/readattrs.ll
new file mode 100644
index 00000000000..7baee998613
--- /dev/null
+++ b/test/Transforms/GVN/readattrs.ll
@@ -0,0 +1,17 @@
+; RUN: opt -gvn -S -o - < %s | FileCheck %s
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @use(i8* readonly nocapture)
+
+define i8 @test() {
+  %a = alloca i8
+  store i8 1, i8* %a
+  call void @use(i8* %a)
+  %b = load i8* %a
+  ret i8 %b
+; CHECK: define i8 @test
+; CHECK: call void @use(i8* %a)
+; CHECK-NEXT: ret i8 1
+}
-- 
2.34.1