http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49012
Summary: weak const optimisations
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: etienne_lorr...@yahoo.fr
With newer GCC compilers,
const struct { int a,b; } mystruct = {15, 0};
int adder (int x) { return x + mystruct.b; };
there isn't any addition performed in adder() because GCC knows that mystruct.b
is null,
and assumes that nobody is initialising mystruct.b is assembler or declaring
mystruct as
non-const in another module.
But if we add the weak attribute to mystruct, GCC-4.4.5 does the addition in
case mystruct
has been pre-loaded by for instance LD_PRELOAD, GCC-4.6 do not do the addition.
const struct { int a,b; } mystruct __attribute__((weak))= {15, 0};
int adder (int x) { return x + mystruct.b; };
LD_PRELOAD=/home/etienne/projet/toolchain/lib/libmpc.so.2:/home/etienne/projet/toolchain/lib/libgmp.so.10
/home/etienne/projet/toolchain/bin/gcc -Os -fomit-frame-pointer -S t.c -o t.s
gives:
.file "t.c"
.text
.globl adder
.type adder, @function
adder:
.LFB0:
.cfi_startproc
movl 4(%esp), %eax
ret
.cfi_endproc
.LFE0:
.size adder, .-adder
.weak mystruct
.section .rodata
.align 4
.type mystruct, @object
.size mystruct, 8
mystruct:
.long 15
.long 0
.ident "GCC: (GNU) 4.6.0"
.section .note.GNU-stack,"",@progbits
