From d7cc8b839cba201b552698646b624da7a79ede8e Mon Sep 17 00:00:00 2001
From: Richard Osborne <richard@xmos.com>
Date: Tue, 18 Sep 2012 09:31:44 +0000
Subject: [PATCH] Fix instcombine to obey requested alignment when merging
 allocas.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164117 91177308-0d34-0410-b5e6-96231b3b80d8
---
 .../InstCombineLoadStoreAlloca.cpp            | 10 +++++---
 .../InstCombine/2012-09-17-ZeroSizedAlloca.ll | 24 +++++++++++++++++++
 2 files changed, 31 insertions(+), 3 deletions(-)
 create mode 100644 test/Transforms/InstCombine/2012-09-17-ZeroSizedAlloca.ll

diff --git a/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
index 6ecb4c52c40..5b6cf4a4a83 100644
--- a/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ b/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -246,12 +246,16 @@ Instruction *InstCombiner::visitAllocaInst(AllocaInst &AI) {
           return &AI;
         }
 
+        // If the alignment of the entry block alloca is 0 (unspecified),
+        // assign it the preferred alignment.
+        if (EntryAI->getAlignment() == 0)
+          EntryAI->setAlignment(
+            TD->getPrefTypeAlignment(EntryAI->getAllocatedType()));
         // Replace this zero-sized alloca with the one at the start of the entry
         // block after ensuring that the address will be aligned enough for both
         // types.
-        unsigned MaxAlign =
-          std::max(TD->getPrefTypeAlignment(EntryAI->getAllocatedType()),
-                   TD->getPrefTypeAlignment(AI.getAllocatedType()));
+        unsigned MaxAlign = std::max(EntryAI->getAlignment(),
+                                     AI.getAlignment());
         EntryAI->setAlignment(MaxAlign);
         if (AI.getType() != EntryAI->getType())
           return new BitCastInst(EntryAI, AI.getType());
diff --git a/test/Transforms/InstCombine/2012-09-17-ZeroSizedAlloca.ll b/test/Transforms/InstCombine/2012-09-17-ZeroSizedAlloca.ll
new file mode 100644
index 00000000000..ba025e92b01
--- /dev/null
+++ b/test/Transforms/InstCombine/2012-09-17-ZeroSizedAlloca.ll
@@ -0,0 +1,24 @@
+; RUN: opt -S -instcombine < %s | FileCheck %s
+
+; When merging zero sized alloca check that requested alignments of the allocas
+; are obeyed.
+
+@x = global i8* null, align 8
+@y = global i8* null, align 8
+
+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-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.8.0"
+
+; CHECK: @f
+; CHECK-NEXT: alloca [0 x i8], align 1024
+; CHECK-NOT: alloca
+; CHECK: ret void
+define void @f() {
+  %1 = alloca [0 x i8], align 1
+  %2 = alloca [0 x i8], align 1024
+  %3 = getelementptr inbounds [0 x i8]* %1, i64 0, i64 0
+  %4 = getelementptr inbounds [0 x i8]* %2, i64 0, i64 0
+  store i8* %3, i8** @x, align 8
+  store i8* %4, i8** @y, align 8
+  ret void
+}