https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67090
Bug ID: 67090
Summary: [5/6 Regression] O2/O3 optimizes values away when
different type pointers are typecasted
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: davispuh at gmail dot com
Target Milestone: ---
GCC 5.2.0 and also latest trunk 6.0 (compiled from git commit 9b6b471, svn
r226241) using O2 or O3 generates invalid code by optimizing value away when
it's pointer is casted to different type pointer.
Here's a testcase, basically cast (double *) to (long *) and (long *) to
(double *)
#include
int main(int argc, char **argv)
{
long l = 0xAA00133700BB00EE;
double d = (double)-2.190302448144169e-106;
volatile int i = 0; // this to prevent optimization, could be also argc-1
printf("%16lX\n%.15e\n", ((long*)&d)[i], ((double*)&l)[i]);
return 0;
}
Compiling it with gcc -O1 or with clang -O3 produces correct output:
AA00133700BB00EE
-2.190302448144169e-106
But when compiling with gcc -O2 (or -O3) produces incorrect output
0
6.952967796700164e-310
When compiled correctly with -O1
0x004004fa48b8ee00bb00. movabs rax, -0x55ffecc8ff44ff12
0x004005044889442418 mov qword [rsp + 0x18], rax
0x00400509f20f100dd700. movsd xmm1, qword [rip + 0xd7] ;
[0x4005e8:8]=0xaa00133700bb00ee
0x00400511f20f114c2410 movsd qword [rsp + 0x10], xmm1
0x00400517c744240c. mov dword [rsp + 0xc], 0
0x0040051f8b54240c mov edx, dword [rsp + 0xc] ;
[0xc:4]=-1 ; 12
0x004005238b44240c mov eax, dword [rsp + 0xc] ;
[0xc:4]=-1 ; 12
0x004005274863d2 movsxd rdx, edx
0x0040052af20f1044d418 movsd xmm0, qword [rsp + rdx*8 + 0x18]
; [0x18:8]=-1 ; 24
0x004005304898 cdqe
0x00400532488b74c410 mov rsi, qword [rsp + rax*8 + 0x10] ;
[0x10:8]=-1 ; 16
0x00400537bfd4054000 mov edi, str._16lX_n_.15e_n;
"%16lX.%.15e." @ 0x4005d4
0x0040053cb80100 mov eax, 1
0x00400541e89afe call sym.imp.printf
;0x004003e0() ; sym.imp.printf
0x00400546b8 mov eax, 0
0x0040054b4883c428 add rsp, 0x28
0x0040054fc3 ret
memory at rsp (0x7ffea6ee18c0)
- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF
0x7ffea6ee18c0 5005 4000 1004 4000 P.@...@.
0x7ffea6ee18d0 ee00 bb00 3713 00aa ee00 bb00 3713 00aa 7...7... //
values here
0x7ffea6ee18e0 9037 5a02 437f .7Z.C...
0x7ffea6ee18f0 c819 eea6 fe7f c819 eea6 fe7f
When compiled incorrectly with -O2
0x00400414bfc4054000 mov edi, str._16lX_n_.15e_n;
"%16lX.%.15e." @ 0x4005c4
0x00400419c744240c. mov dword [rsp + 0xc], 0
0x00400421486354240c movsxd rdx, dword [rsp + 0xc] ;
[0xc:4]=-1 ; 12
0x00400426486344240c movsxd rax, dword [rsp + 0xc] ;
[0xc:4]=-1 ; 12
0x0040042bf20f1044d410 movsd xmm0, qword [rsp + rdx*8 + 0x10]
; [0x10:8]=-1 ; 16
0x00400431488b74c418 mov rsi, qword [rsp + rax*8 + 0x18] ;
[0x18:8]=-1 ; 24
0x00400436b80100 mov eax, 1
0x0040043be8a0ff call sym.imp.printf
;0x004003e0() ; sym.imp.printf
0x0040044031c0 xor eax, eax
0x004004424883c428 add rsp, 0x28
0x00400446c3 ret
memory at rsp (0x7ffd8ac419d0)
- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF
0x7ffd8ac419d0 4005 4000 5004 4000 @.@.P.@.
0x7ffd8ac419e0 d01a c48a fd7f //
values aren't here
0x7ffd8ac419f0 90b7 be39 047f ...9
0x7ffd8ac41a00 d81a c48a fd7f d81a c48a fd7f
Running x86_64 Arch Linux, gcc -v
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-unknown-linux-gnu/6.0.140195.9b6b471/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: /mnt/AUR/gcc-multilib-trunk-git/src/gcc/configure
--prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-clocale=gnu --disable-libstdcxx-pch
--disable-libssp --enable-gnu-unique-object --enable-linker-build-id
--enable-cl
