https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64541
Bug ID: 64541
Summary: .fre1 pass optimization failure
Product: gcc
Version: 5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: skvadrik at gmail dot com
Created attachment 34403
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34403&action=edit
1.c 2.c 1.c.028t.esra 2.c.028t.esra 1.c.030t.fre1 2.c.030t.fre1 1.s 2.s
compile_gcc.sh
Files 1.c and 2.c in attach are equivalent from C/C++ standpoint:
$ diff 1.c 2.c
3c3,5
< return *(*q = ++*p);
---
> ++*p;
> *q = *p;
> return **p;
Compiled with gcc-5.0.0, disassembled with objdump (GNU Binutils) 2.24
(see full script compile_gcc.sh in attach).
For 1.c gcc generates better code than for 2.c
(compare 1.s vs 2.s in attach).
I looked at GIMPLE optimization dumps (-fdump-tree-all) and found that
up to *.029t.ealias pass dumps differ insignificantly (attached
*.028t.esra dumps as they are shorter), but .030t.fre1 pass fails to
reduce intermediate variable '_9' in the second case (attached *.030t.fre
dumps).
Looks like a bug in full redundancy elimination.
