https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85593
Bug ID: 85593
Summary: GCC on ARM allocates R3 for local variable when
calling naked function with O2 optimizations enabled
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: inline-asm
Assignee: unassigned at gcc dot gnu.org
Reporter: austinpmorton at gmail dot com
Target Milestone: ---
Created attachment 44048
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44048&action=edit
example code triggering bug
When calling a naked function declared in the same translation unit which
includes inline assembly that modifies registers which are not preserved across
function calls according to the arm abi (R0-R3), GCC 5+ can incorrectly
allocate these registers for local variables in the caller and expect them to
persist between function calls.
* -O2 must be enabled to observe this behavior.
* the function must be declared in the same translation unit, if declared
extern gcc correctly allocates preserved registers for its locals
* GCC 4.6.4 correctly allocates R4 in the example code given
* GCC 5.4, and all higher versions tested incorrectly allocates R3 in the
example code given
* I have built a clean copy of GCC trunk and observed this issue as well
note: this issue is present on all arm triplets I have tested, not just
arm-none-eabi.
compile bug.c with the following options to observe the bug
arm-none-eabi-gcc -O2 -DBUG -S bug.c -o -
compile bug.c with the following options to observe the correct behavior with
an extern declaration
arm-none-eabi-gcc -O2 -S bug.c -o -
note that in the non-working compiler versions gcc attempts to load a value
from r3 between calls to test2, whereas in the working compiler versions gcc
will produce identical code save for using r4.
for convenience sake, here is the example shown in working vs nonworking
compiler versions on godbolt:
https://godbolt.org/g/kMSn8W
