RE: Pb with G++ 3.4.2 MinGW and inline assembly

["John Yates" <jyates at netezza dot com>]

You have not properly described the side effects of either the
movsl instruction nor the rep prefix.  The values loaded by
the "S", "D", and "c" constraints are all clobbered by the
time the instruction sequence completes.  If you do not inform
the code generator of this it assumes that the registers remain
unchanged.

/john

-----Original Message-----
From: Studious Apprentice [mailto:apprentice_99@hotmail.com]
Sent: Monday, February 06, 2006 3:32 AM
To: gcc-help@gcc.gnu.org
Subject: Pb with G++ 3.4.2 MinGW and inline assembly


Hello !

G++ seems to use a register which should have been clobbered with inline 
assembly.

I am using the following assembly code (80x86) to inline a fast memcpy for 
variable dword lengths arguments. The program works well in debug mode :"g++ 
-g" but not in optimized mode : "g++ -g -Wall -O2 -DNDEBUG". The compilation 
is fine but GCC reuses the EDI register after inlining this routine, which 
hangs my program. It is no longer possible to explicitly indicate a register 
as clobbered when it is already used as a parameter, so I do not see any 
workaround for what looks a bug.

Any idea ?

Thanks in advance,

Charles

inline void fast_memcopy
(void * where, void const * from, std::size_t times)
{
	__asm__ __volatile__
		(
		"cld\n\t"
		"rep\n\t"
		"movsl"
		:
	: "S"(from), "D"(where), "c"(times)
		: "memory"
		);
}