/* When a function has been defined using the 'noreturn' attribute,
* there is no reason to save the callee-saved registers, mainly
* because the function is not going to return to the caller.
*
* This can save a few bytes of code generation for embedded-type
* applications which may use 'noreturn' functions to restart, for
* example, by jumping to a different address rather than returning to
* the caller.
*
* gcc information:
*
* Using built-in specs.
* Target: x86_64-redhat-linux
*
* Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
* --infodir=/usr/share/info --enable-shared
* --enable-threads=posix --enable-checking=release
* --with-system-zlib --enable-__cxa_atexit
* --disable-libunwind-exceptions
--enable-libgcj-multifile
* --enable-languages=c,c++,objc,obj-c++,java,fortran,ada
* --enable-java-awt=gtk --disable-dssi --enable-plugin
*
--with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre
* --with-cpu=generic --host=x86_64-redhat-linux
* Thread model: posix
* gcc version 4.1.2 20071124 (Red Hat 4.1.2-42)
*
* compilation & disassembly information:
*
* gcc -c -O3 -o noreturn.o noreturn.c
* objdump --disassemble --reloc noreturn.o
*
* Notice in the disassembly of 'return_to_caller' that the
* registers pushed in the preamble are restored before
* returning. But, in the 'no_return_to_caller', they are not
* restored; since they are no restored, there is no reason to
* even save them.
*/
#include <stdlib.h>
extern unsigned array[1024][1024][1024];
extern unsigned value(int i, int j, int k);
#define LOOP_BODY \
do { \
unsigned i, j, k; \
for (i = 1024; i > 0; --i) { \
for (j = 1024; j > 0; --j) { \
for (k = 1024; k > 0; --k) { \
array[i - 1][j - 1][k - 1] = value(i, j, k); \
} \
} \
} \
} while (0)
void
return_to_caller(int a, int b, int c, int d)
{
LOOP_BODY;
}
void __attribute__((noreturn))
no_return_to_caller(int a, int b, int c, int d)
{
LOOP_BODY;
while (1);
}
--
Summary: gcc 4.2.1 and above: No need to save called-saved
registers in 'noreturn' function
Product: gcc
Version: 4.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: thutt at vmware dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38534
