PNaCl: Fix FlattenGlobals to correctly handle implicitly-aligned variables

If a global variable has no "align" attribute, it must be aligned
based on its type.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3437
TEST=flatten-globals.ll

Review URL: https://codereview.chromium.org/15359006

Author: Mark Seaborn

lib/Transforms/NaCl/FlattenGlobals.cpp

@@ -255,8 +255,8 @@ bool FlattenGlobals::runOnModule(Module &M) {
       continue;
     Modified = true;
 
-    uint64_t Size = DL.getTypeAllocSize(
-        Global->getType()->getPointerElementType());
+    Type *GlobalType = Global->getType()->getPointerElementType();
+    uint64_t Size = DL.getTypeAllocSize(GlobalType);
     Constant *NewInit;
     Type *NewType;
     if (Global->hasInitializer()) {
@@ -281,6 +281,8 @@ bool FlattenGlobals::runOnModule(Module &M) {
         NewInit, "", Global,
         Global->getThreadLocalMode());
     NewGlobal->copyAttributesFrom(Global);
+    if (NewGlobal->getAlignment() == 0)
+      NewGlobal->setAlignment(DL.getPrefTypeAlignment(GlobalType));
     NewGlobal->setExternallyInitialized(Global->isExternallyInitialized());
     NewGlobal->takeName(Global);
     Global->replaceAllUsesWith(

test/Transforms/NaCl/flatten-globals.ll

@@ -91,6 +91,26 @@ target datalayout = "p:32:32:32"
 ; CHECK: @aligned_var = global [4 x i8] c"\04\01\00\00", align 8
 
 
+; Check alignment handling
+
+@implicit_alignment_i32 = global i32 zeroinitializer
+; CHECK: @implicit_alignment_i32 = global [4 x i8] zeroinitializer, align 4
+
+@implicit_alignment_double = global double zeroinitializer
+; CHECK: @implicit_alignment_double = global [8 x i8] zeroinitializer, align 8
+
+; FlattenGlobals is not allowed to increase the alignment of the
+; variable when an explicit section is specified (although PNaCl does
+; not support this attribute).
+@lower_alignment_section = global i32 0, section "mysection", align 1
+; CHECK: @lower_alignment_section = global [4 x i8] zeroinitializer, section "mysection", align 1
+
+; FlattenGlobals could increase the alignment when no section is
+; specified, but it does not.
+@lower_alignment = global i32 0, align 1
+; CHECK: @lower_alignment = global [4 x i8] zeroinitializer, align 1
+
+
 ; Check handling of global references
 
 @var1 = external global i32