[LLVMdev] RFA: Alignment of Strings
Bill Wendling
isanbard at gmail.com
Tue Jun 2 18:33:52 PDT 2009
One of our developers pointed out that GCC generates something akin to
this on Darwin:
$ cat t.c
__private_extern__ void bar(const char *format, ...)
__attribute__ ((format (printf, 1, 2)));
void foo(const char *i, const char *b) {
bar("this is some error message that is kind of longish.");
}
$ gcc -arch i386 -S -o - t.c -Os
.cstring
.align 2
LC0:
.ascii "this is some error message that is kind of longish.\0"
...
LLVM on the other hand generates this on Darwin:
$ /Developer/usr/bin/llvm-gcc -arch i386 -S -o - t.c -Os
...
.section __TEXT,__cstring,cstring_literals
.align 4
LC:
.asciz "this is some error message that is kind of longish."
Note that the alignment in LLVM is greater than it is for GCC. When I
look at the alignment of the string in GDB, the front-end is saying
that it's 8, which would be '.align 3'. I have a patch below that sets
the alignment explicitly for STRING_CST types.
My question is two-fold:
* Is the patch good? I don't *think* that it will break anything else
in the front-end or for different platforms, but one never knows, and,
* What magick is GCC performing to get the '.align 2' instead of
'.align 3', which I would expect? Is this something we can/should
replicate in LLVM?
-bw
Index: llvm-gcc.src/gcc/llvm-convert.cpp
===================================================================
--- llvm-gcc.src/gcc/llvm-convert.cpp (revision 72733)
+++ llvm-gcc.src/gcc/llvm-convert.cpp (working copy)
@@ -7418,6 +7418,8 @@
TAI ?
TAI->getStringConstantPrefix() :
".str", TheModule);
+ GV->setAlignment(TYPE_ALIGN(TREE_TYPE(exp)));
+
if (SlotP) *SlotP = GV;
#ifdef LLVM_CSTRING_SECTION
// For Darwin, try to put it into the .cstring section.
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