https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89354
Bug ID: 89354
Summary: Combine pass yields wrong code with -O2 and -msse2 for
32bit target
Product: gcc
Version: 6.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: d.sukhonin at gmail dot com
Target Milestone: ---
Created attachment 45723
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=45723&action=edit
Proposed patch for gcc 6.3 including a test
There is a problem with compiling this code (options -O2 -msse2 -m32):
$ gcc-6 -m32 -O2 -msse2 /src/gcc/gcc/testsuite/gcc.target/i386/sse2-extz_di.c
&& ./a.out
Aborted
#define BIT33 0x100000000ULL
#include <stdint.h>
static uint64_t const MASK33 = (1ULL << 33) - 1;
static uint64_t qword = 0;
static uint64_t
get_low33 (void)
{
return qword & MASK33;
}
static void
__attribute__((noinline))
set_bit33 (void)
{
qword = (qword & ~MASK33) | BIT33;
}
static void
main (int, char**)
{
set_bit33 ();
if (get_low33 () != BIT33))
abort ();
}
Investigation showed, that during combine pass for set_bit33 function wrong RTL
is yielded: the second operand of ior is truncated into dword, i.e. 0, and the
bit 33 is never switched on.
I have a fix attached. It does not allow narrowing while matching zero_extract
insn.
That is, before the fix the optimizer was dealing with this RTL inst (Note,
zero_extract:SI):
Trying 5, 6, 7 -> 8:
Failed to match this instruction:
(set (zero_extract:SI (mem/c:DI (plus:SI (reg:SI 87)
(const:SI (unspec:SI [
(symbol_ref:SI ("qword") [flags 0x2] <var_decl
0x7f6921b205a0 qword>)
] UNSPEC_GOTOFF))) [3 qword+0 S8 A64])
(const_int 33 [0x21])
(const_int 0 [0]))
(const_int 0 [0]))
Now zero_extract has DI mode because the inner instruction mem has larger, DI,
mode size:
Trying 5, 6, 7 -> 8:
Failed to match this instruction:
(set (zero_extract:DI (mem/c:DI (symbol_ref:SI ("qword") [flags 0x2] <var_decl
0x7f7c3083c480 qword>) [3 qword+0 S8 A64])
(const_int 33 [0x21])
(const_int 0 [0]))
(const_int 4294967296 [0x100000000]))
References:
* get_best_reg_extraction_insn is called here
https://github.com/gcc-mirror/gcc/blob/master/gcc/combine.c#L7819
* zero_extract is tried out
https://github.com/gcc-mirror/gcc/blob/master/gcc/combine.c#L7983
$ gcc-6 -v
Using built-in specs.
COLLECT_GCC=gcc-6
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 6.3.0-18+deb9u1'
--with-bugurl=file:///usr/share/doc/gcc-6/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-6 --program-prefix=x86_64-linux-gnu- --enable-shared
--enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/
--enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie
--with-system-zlib --disable-browser-plugin --enable-java-awt=gtk
--enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-6-amd64/jre
--enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-6-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-6-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--with-target-system-zlib --enable-objc-gc=auto --enable-multiarch
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 6.3.0 20170516 (Debian 6.3.0-18+deb9u1)
It is reproduced on all versions >= 6.3.
I didn't test on older ones.
