PR 49169: testing the alignment of a function
Richard Sandiford
rdsandiford@googlemail.com
Wed Jun 29 08:40:00 GMT 2011
"H.J. Lu" <hjl.tools@gmail.com> writes:
> This caused:
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49545
Sorry for the breakage. I should obviously have tested on x86_64 as well.
To recap, there are (at least) two concepts of what "the address of X"
can mean. It can mean (a) the address at which X is actually located
or (b) the value to which symbol X resolves. The problem is that these
two are different for things like Thumb and MIPS16 functions.
The DECL_ALIGN on a function should be (a). For example, if we have:
void __attribute__((aligned(16))) foo (void) { ... }
then foo() really ought to start on a 16-byte boundary, even for Thumb.
However, most parts of GCC want (b). In particular, if we want to know
the alignment of an ADDR_EXPR, or the alignment of a tree or RTL memory
reference, the address is going to come from symbol resolution.
As far as I can tell, all uses of get_object_alignment* want (b).
In particular, get_object_alignment* isn't used for __alignof__,
which might have been one case where (a) made more sense.
(Any thoughts on what __alignof__ should mean here? Either way,
it's a separate patch.)
So DECL_ALIGN gives (a) rather than (b). But as the PR shows,
there are cases where we know (and need to know) that (b) >= 2 bytes,
namely whenever TARGET_PTRMEMFUNC_VBIT_LOCATION == ptrmemfunc_vbit_in_pfn.
This patch uses that to decide whether &function has 1 or 2 bytes of
alignment.
Note that this isn't just about ARM and MIPS. Some targets define
FUNCTION_BOUNDARY in a way that depends on optimisation flags,
whereas get_object_alignment previously treated it as an ABI property.
It's definitely arguable that those targets are buggy and should be
using align_functions instead, but the docs aren't really clear.
That's why I'm still not going for a target hook at this stage.
It's just too hard to say in general whether a port's current
FUNCTION_BOUNDARY is guaranteed by the ABI or not, so I think
the default definition of any hook would still be the condition
that I've used here. I think it makes sense to leave it like
this until someone is motivated to guarantee greater alignment.
Tested on x86_64-linux-gnu and arm-linux-gnueabi. OK to install?
Richard
gcc/
PR tree-optimization/49545
* builtins.c (get_object_alignment_1): Update function comment.
Do not use DECL_ALIGN for functions, but test
TARGET_PTRMEMFUNC_VBIT_LOCATION instead.
* fold-const.c (get_pointer_modulus_and_residue): Don't check
for functions here.
* tree-ssa-ccp.c (get_value_from_alignment): Likewise.
gcc/testsuite/
* gcc.dg/torture/pr49169.c: Restrict to ARM and MIPS targets.
Index: gcc/builtins.c
===================================================================
--- gcc/builtins.c 2011-06-28 14:06:19.000000000 +0100
+++ gcc/builtins.c 2011-06-29 09:07:00.000000000 +0100
@@ -264,8 +264,15 @@ called_as_built_in (tree node)
return is_builtin_name (name);
}
-/* Return the alignment in bits of EXP, an object.
- Don't return more than MAX_ALIGN no matter what. */
+/* Compute values M and N such that M divides (address of EXP - N) and
+ such that N < M. Store N in *BITPOSP and return M.
+
+ Note that the address (and thus the alignment) computed here is based
+ on the address to which a symbol resolves, whereas DECL_ALIGN is based
+ on the address at which an object is actually located. These two
+ addresses are not always the same. For example, on ARM targets,
+ the address &foo of a Thumb function foo() has the lowest bit set,
+ whereas foo() itself starts on an even address. */
unsigned int
get_object_alignment_1 (tree exp, unsigned HOST_WIDE_INT *bitposp)
@@ -287,7 +294,21 @@ get_object_alignment_1 (tree exp, unsign
exp = DECL_INITIAL (exp);
if (DECL_P (exp)
&& TREE_CODE (exp) != LABEL_DECL)
- align = DECL_ALIGN (exp);
+ {
+ if (TREE_CODE (exp) == FUNCTION_DECL)
+ {
+ /* Function addresses can encode extra information besides their
+ alignment. However, if TARGET_PTRMEMFUNC_VBIT_LOCATION
+ allows the low bit to be used as a virtual bit, we know
+ that the address itself must be 2-byte aligned. */
+ if (TARGET_PTRMEMFUNC_VBIT_LOCATION == ptrmemfunc_vbit_in_pfn)
+ align = 2 * BITS_PER_UNIT;
+ else
+ align = BITS_PER_UNIT;
+ }
+ else
+ align = DECL_ALIGN (exp);
+ }
else if (CONSTANT_CLASS_P (exp))
{
align = TYPE_ALIGN (TREE_TYPE (exp));
Index: gcc/fold-const.c
===================================================================
--- gcc/fold-const.c 2011-06-28 14:06:19.000000000 +0100
+++ gcc/fold-const.c 2011-06-29 08:59:02.000000000 +0100
@@ -9216,8 +9216,7 @@ get_pointer_modulus_and_residue (tree ex
*residue = 0;
code = TREE_CODE (expr);
- if (code == ADDR_EXPR
- && TREE_CODE (TREE_OPERAND (expr, 0)) != FUNCTION_DECL)
+ if (code == ADDR_EXPR)
{
unsigned int bitalign;
bitalign = get_object_alignment_1 (TREE_OPERAND (expr, 0), residue);
Index: gcc/tree-ssa-ccp.c
===================================================================
--- gcc/tree-ssa-ccp.c 2011-06-28 14:06:19.000000000 +0100
+++ gcc/tree-ssa-ccp.c 2011-06-29 08:59:02.000000000 +0100
@@ -520,10 +520,6 @@ get_value_from_alignment (tree expr)
val = bit_value_binop (PLUS_EXPR, TREE_TYPE (expr),
TREE_OPERAND (base, 0), TREE_OPERAND (base, 1));
else if (base
- /* ??? While function decls have DECL_ALIGN their addresses
- may encode extra information in the lower bits on some
- targets (PR47239). Simply punt for function decls for now. */
- && TREE_CODE (base) != FUNCTION_DECL
&& ((align = get_object_alignment (base, BIGGEST_ALIGNMENT))
> BITS_PER_UNIT))
{
Index: gcc/testsuite/gcc.dg/torture/pr49169.c
===================================================================
--- gcc/testsuite/gcc.dg/torture/pr49169.c 2011-06-29 08:58:51.000000000 +0100
+++ gcc/testsuite/gcc.dg/torture/pr49169.c 2011-06-29 09:00:41.000000000 +0100
@@ -1,3 +1,5 @@
+/* { dg-do compile { target { arm*-*-* || mips*-*-* } } } */
+
#include <stdlib.h>
#include <stdint.h>
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