Hi,
I have noticed an interesting behaviour based on the memset builtin test on
4.8.1. If you compile memset-lib.i with -O3 you get an infinite recursion.
$ cat memset-lib.i
# 1
"/home/pmatos/work/fp_gcc-25x/gcc/testsuite/gcc.c-torture/execute/builtins/memset-lib.c"
# 1 "<command-line>"
# 1
"/home/pmatos/work/fp_gcc-25x/gcc/testsuite/gcc.c-torture/execute/builtins/memset-lib.c"
# 1
"/home/pmatos/work/fp_gcc-25x/gcc/testsuite/gcc.c-torture/execute/builtins/lib/memset.c"
1
extern void abort (void);
extern int inside_main;
__attribute__ ((__noinline__))
void *
memset (void *dst, int c, long unsigned int n)
{
while (n-- != 0)
n[(char *) dst] = c;
if (inside_main && n < 2)
abort ();
return dst;
}
# 1
"/home/pmatos/work/fp_gcc-25x/gcc/testsuite/gcc.c-torture/execute/builtins/memset-lib.c"
2
$ ~/work/tmp/gcc_4.8.1-build/gcc/cc1 -O3 memset-lib.i -o-
.file "memset-lib.i"
memset
Analyzing compilation unit
Performing interprocedural optimizations
<*free_lang_data> <visibility> <early_local_cleanups> <*free_inline_summary>
<whole-program> <profile_estimate> <cp> <inline> <pure-const>
<static-var>Assembling functions:
memset .text
.p2align 4,,15
.globl memset
.type memset, @function
memset:
.LFB0:
.cfi_startproc
testq %rdx, %rdx
je .L6
subq $8, %rsp
.cfi_def_cfa_offset 16
movsbl %sil, %esi
call memset
addq $8, %rsp
.cfi_def_cfa_offset 8
ret
.p2align 4,,10
.p2align 3
.L6:
movq %rdi, %rax
ret
.cfi_endproc
.LFE0:
.size memset, .-memset
.ident "GCC: (GNU) 4.8.1"
.section .note.GNU-stack,"",@progbits
So, it seems the GCC is smart enough to detect the pattern for memset but once
it replaces the loop into the function a recursion is created because the
function its replacing the loop in is called memset itself.
Interestingly GCC doesn't actually fail the test, probably due to interactions
with other files and functions but this compilation unit does generate
incorrect output so I think there might be a bug lurking here.
Another thing I cannot grasp is that the test for inside_main completely
disappears. We do know n <= 2 after the loop but we have no information for
inside_main so I would assume we still need to test inside_main to know if we
should abort.
What do you think?
Paulo Matos
