Re: gcc-nm behavior with -flto

[Arvind Sankar <nivedita@alum.mit.edu>]

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.