http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58073
Bug ID: 58073
Summary: Suboptimal optimisation of ((x & 0x70) == 0x00 && (x &
0x70) == 0x10 && (x & 0x70) == 0x20) on x86_64
Product: gcc
Version: 4.8.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: dhowells at redhat dot com
Created attachment 30605
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30605&action=edit
Demonstration source code
When the attached demo code is compiled with gcc-4.8.1, two of the cases
optimise fine and the third case is optimised suboptimally - probably because
it initially matches the optimisation for the second case.
Going through the cases individually for 'shift' being 4:
(1) return (mask(d) == (0x0 << shift));
This is rendered as a single TEST instruction in x86_64 asm:
0: f6 07 70 testb $0x70,(%rdi)
3: 0f 94 c0 sete %al
6: c3 retq
which is good.
(2) return (mask(d) == (0x0 << shift) ||
mask(d) == (0x1 << shift));
This is also rendered as a single TEST instruction:
10: f6 07 60 testb $0x60,(%rdi)
13: 0f 94 c0 sete %al
16: c3 retq
which is again good. The problem comes with the third case:
(3) return (mask(d) == (0x0 << shift) ||
mask(d) == (0x1 << shift) ||
mask(d) == (0x2 << shift));
This is rendered as:
20: 8b 17 mov (%rdi),%edx
22: b8 01 00 00 00 mov $0x1,%eax
27: f6 c2 60 test $0x60,%dl
2a: 74 09 je 35 <foo3+0x15>
2c: 83 e2 70 and $0x70,%edx
2f: 83 fa 20 cmp $0x20,%edx
32: 0f 94 c0 sete %al
35: f3 c3 repz retq
which is odd. I would expect the thing to be turned into MOV, AND, CMP, SETE,
RETQ since the numbers it is checking for lie adjacent to each other, starting
from zero.
I think what has happened is that the first two comparisons matched the
optimisation for case (2) - resulting in three extra instructions.
The compilation command line was:
gcc -O2 -c foo.c -Wall && objdump -d foo.o
The compiler version:
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.8.1/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla
--enable-bootstrap --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-gnu-unique-object
--enable-linker-build-id --with-linker-hash-style=gnu
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada,go,lto --enable-plugin
--enable-initfini-array --enable-java-awt=gtk --disable-dssi
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre
--enable-libgcj-multifile --enable-java-maintainer-mode
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libjava-multilib
--with-isl=/builddir/build/BUILD/gcc-4.8.1-20130603/obj-x86_64-redhat-linux/isl-install
--with-cloog=/builddir/build/BUILD/gcc-4.8.1-20130603/obj-x86_64-redhat-linux/cloog-install
--with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 4.8.1 20130603 (Red Hat 4.8.1-1) (GCC)
as supplied by Fedora: gcc-4.8.1-1.fc19.x86_64
