Fix a theoretical problem (not seen in the wild): if different instances of a

weak variable are compiled by different compilers, such as GCC and LLVM, while
LLVM may increase the alignment to the preferred alignment there is no reason to
think that GCC will use anything more than the ABI alignment.  Since it is the
GCC version that might end up in the final program (as the linkage is weak), it
is wrong to increase the alignment of loads from the global up to the preferred
alignment as the alignment might only be the ABI alignment.

Increasing alignment up to the ABI alignment might be OK, but I'm not totally
convinced that it is.  It seems better to just leave the alignment of weak
globals alone.

llvm-svn: 145413

1 files changed, 10 insertions, 3 deletions

diff --git a/llvm/test/Transforms/InstCombine/align-external.ll b/llvm/test/Transforms/InstCombine/align-external.ll
index 6e8ad87f19e..d4a5d429912 100644
--- a/llvm/test/Transforms/InstCombine/align-external.ll
+++ b/llvm/test/Transforms/InstCombine/align-external.ll
@@ -1,7 +1,7 @@
 ; RUN: opt < %s -instcombine -S | FileCheck %s
 
-; Don't assume that external global variables have their preferred
-; alignment. They may only have the ABI minimum alignment.
+; Don't assume that external global variables or those with weak linkage have
+; their preferred alignment. They may only have the ABI minimum alignment.
 
 ; CHECK: %s = shl i64 %a, 3
 ; CHECK: %r = or i64 %s, ptrtoint (i32* @A to i64)
@@ -11,7 +11,7 @@
 target datalayout = "-i32:8:32"
 
 @A = external global i32
-@B = external global i32
+@B = weak_odr global i32 0
 
 define i64 @foo(i64 %a) {
   %t = ptrtoint i32* @A to i64
@@ -20,3 +20,10 @@ define i64 @foo(i64 %a) {
   %q = add i64 %r, 1
   ret i64 %q
 }
+
+define i32 @bar() {
+; CHECK: @bar
+  %r = load i32* @B, align 1
+; CHECK: align 1
+  ret i32 %r
+}