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Re: gcc-nm behavior with -flto
- From: Arvind Sankar <nivedita at alum dot mit dot edu>
- To: gcc-help at gcc dot gnu dot org, xry111 at mengyan1223 dot wang
- Cc: binutils at sourceware dot org
- Date: Wed, 5 Feb 2020 14:40:22 -0500
- Subject: Re: gcc-nm behavior with -flto
- References: <20200204205851.GA3873578@rani.riverdale.lan>
On 2020-02-05 10:42 +0800, Xi Ruoyao wrote:
> I can't reproduce this.
>
> $ cat test1.c
> void text(void) {}
> int data = 1;
> int bss = 0;
> int common;
> $ cat test2.c
> #include <assert.h>
>
> extern void text(void);
> extern int data;
> extern int bss;
> extern int common;
>
> int main()
> {
> assert(bss == 0 && data == 1);
> return 0;
> }
> $ cc test1.c test2.c -flto -fno-common
> $ ./a.out
> $
>
> Did you forgot the "extern" in test2.c for "int common;"? "int common;" is a
> tentative definition, equivalent to "int common = 0;", in ANSI C. "-fcommon" is
> a workaround enabled by default to resolve same tentative definitions in
> multiple files. With "-fno-common" there is no such a workaround so the linker
> will complain.
Hi Xi, no, that's not the issue. The actual files as used in configure
are shown below. Because of nm showing the variable as a "T" symbol,
conftest.c got generated with a function declaration for nm_test_var,
instead of the expected variable declaration.
This causes the configure test to fail with
conftest.cpp:6:12: error: variable 'nm_test_var' redeclared as function
Without -fno-common, nm_test_var shows up as a "C" symbol, so it gets it
right for the configure test, but it would be wrong if/when it's actually
used for .bss or .data symbols.
I found that there is a bugzilla entry for this
https://sourceware.org/bugzilla/show_bug.cgi?id=25355 already.
I also got to wondering why this doesn't break gcc-10 LTO bootstrap,
which also uses libtool, and it turns out its because
lt__PROGRAM_LTX_preloaded_symbols[] is declared as a file-scope
constant, which in C++ defaults to static linkage. We use xg++ as the
compiler for the configure test, and at -O1 and above, conftest.c gets
optimized into nothing (i.e. no symbol references remain) and so the
error doesn't trigger.
conftest.c:
#ifdef __cplusplus
extern "C" {
#endif
extern int nm_test_func();
extern int nm_test_var(); /* This is incorrectly a function declaration */
/* The mapping between symbol names and symbols. */
const struct {
const char *name;
void *address;
}
lt__PROGRAM__LTX_preloaded_symbols[] =
{
{ "@PROGRAM@", (void *) 0 },
{"nm_test_func", (void *) &nm_test_func},
{"nm_test_var", (void *) &nm_test_var},
{0, (void *) 0}
};
/* This works around a problem in FreeBSD linker */
#ifdef FREEBSD_WORKAROUND
static const void *lt_preloaded_setup() {
return lt__PROGRAM__LTX_preloaded_symbols;
}
#endif
#ifdef __cplusplus
}
#endif
conftstm.c:
#ifdef __cplusplus
extern "C" {
#endif
char nm_test_var;
void nm_test_func(void);
void nm_test_func(void){}
#ifdef __cplusplus
}
#endif
int main(){nm_test_var='a';nm_test_func();return(0);}
PS: I didn't receive your e-mail but saw it on gcc-help archive. Not
sure why.