The info pages give something like
__asm__ ("xxx" : "=X" (sum));
as an example of adding a hint for gcc that the instruction clobbers "sum".
However if the variable is an array element, gcc will clobber the variable for
you (in case you forgot) by generating a move from a random register to the
variable. I don't know if liveness analysis is done on element basis for
arrays, but it doesn't generate warnings either.
Similarly for "=X" but not "=m" or "=r".
For example on x86,
__asm__ ("outl %%eax, (%%dx)" : "=X" (params[1]));
to indicate that the IO causes params[1] to be overwritten, results in:
400532: ef out %eax,(%dx)
400533: 89 45 f4 mov %eax,-0xc(%rbp)
with all -O levels.
Similarly on ARM a move from r3 is generated that overwrites the array element.
--
Summary: asm with a "=X" output overwrites the output
Product: gcc
Version: 4.3.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: balrogg at gmail dot com
GCC build triplet: x86_64-linux-gnu, arm-linux-gnueabi
GCC host triplet: x86_64-linux-gnu, arm-linux-gnueabi
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39375
