https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102306
Bug ID: 102306
Summary: Volatile pointer dereferenced twice
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: cederman at gaisler dot com
CC: ebotcazou at libertysurf dot fr, segher at kernel dot
crashing.org
Target Milestone: ---
Created attachment 51448
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51448&action=edit
Test case
The following code (full case in attachment) generates two loads from the
volatile "a" pointer on SPARC:
C
char b = *(volatile unsigned char *)a;
if (!b)
return;
Asm
sethi %hi(a), %g1
ld [%g1+%lo(a)], %g1
ldub [%g1], %g2
ldub [%g1], %g1
cmp %g1, 0
The code is intended to read a memory mapped register that will change after a
read, so two reads will give the wrong value to the comparison.
A bisect showed that this started to happen after "c4c5ad1d6d1e1e1fe7a:
combine: Allow combining two insns to two insns".
The compiler (on commit fc4a29c0781186269dc, latest master at the time) was
configured with:
configure --target=sparc-gaisler-elf --enable-languages=c --disable-nls
--disable-libmudflap --disable-libssp --enable-version-specific-runtime-libs
--disable-fixed-point --disable-decimal-float --disable-shared
and the test was compiled with:
sparc-gaisler-elf -mcpu=v8 -O2
I do not know if this is a generic problem or a SPARC specific one.
