https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70184
Bug ID: 70184
Summary: Exlpicit register variables overwritten by conversion
libcall
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: inline-asm
Assignee: unassigned at gcc dot gnu.org
Reporter: ktkachov at gcc dot gnu.org
Target Milestone: ---
Target: arm
Consider the testcase:
void foo(unsigned a, float b)
{
unsigned c = (unsigned) b;
register unsigned r0 __asm__("r0") = a;
register unsigned r1 __asm__("r1") = c;
__asm__ volatile( "str %[r1], [%[r0]]\n"
:
: [r0] "r" (r0),
[r1] "r" (r1));
}
Compile for an arm target with (for example) -Os -mfloat-abi=soft
-march=armv7-a -marm.
This generates:
foo:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
push {r4, lr}
mov r0, r1
bl __aeabi_f2uiz
mov r1, r0
.syntax divided
@ 7 "foo.c" 1
str r1, [r0]
@ 0 "" 2
.syntax unified
pop {r4, pc}
The register r0 holding 'a' gets overwritten as part of the setup for the
rounding libcall so when it gets tied to the variable 'r0' expecting it to hold
'a' it gets garbage instead.
This testcase is warned about in:
https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html#Local-Register-Variables
but the code above explicitly follows the workaround suggested on that page so
I'd expect it to work.
Is this indeed a supported usecase of local register variables? and if so, is
there a workaround for this bug?
