https://bugs.llvm.org/show_bug.cgi?id=45585
Bug ID: 45585
Summary: Assembler operand constraints are ineffective with
global register variables
Product: clang
Version: 9.0
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: C
Assignee: unassignedclangb...@nondot.org
Reporter: ubiz...@gmail.com
CC: blitzrak...@gmail.com, dgre...@apple.com,
erik.pilking...@gmail.com, llvm-bugs@lists.llvm.org,
richard-l...@metafoo.co.uk
Following testcase:
--cut here--
register unsigned long current_stack_pointer asm("rsp");
void
foo (void)
{
asm volatile ("# %0" :: "a" (current_stack_pointer));
}
--cut here--
compiles on x86 with -O2 to:
#APP
# %rsp
#NO_APP
Please note that the asm uses input operand constraint "a" that would allow
only %rax register.
GCC gets this right and reloads %rsp to %rax:
movq %rsp, %rax
#APP
# %rax
#NO_APP
If the constraint is changed to "r", which allows all general registers,
including %rsp, GCC generates
#APP
# %rsp
#NO_APP
I'd like to argue that the compiler should reload the input argument, so that
the asm input operand constraint is always respected.
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