http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59615
Bug ID: 59615
Summary: "asm goto" output or at least clobbered operands
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: hpa at zytor dot com
There are a number of uses of "asm goto" which requires at least on dyadic
architectures like x86 the ability to clobber an operand. However, the current
definition of "asm goto" doesn't allow any output operands, and the clobber
list only works on specific registers (which are generally not allowed to be
inputs, anyway.)
For example:
asm goto("add %1,%0 ; "
"jc %l[t_err] ; "
"cmp %2,%0 ; "
"ja %l[t_err] ; "
: "+r" (addr)
: "g" (size), "g" (limit)
: : t_err);
return false;
t_err:
return true;
This is part of a proposed implementation of access_ok(), a limit test with a
guard for wraparound, used heavily in the Linux kernel. There needs to be a
way to communicate to gcc that "addr" is clobbered by the first instruction.
