https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71372
Bug ID: 71372
Summary: GCC Optimization "tree-dse" does not respect access to
volatile* upon explicit cast.
Product: gcc
Version: 6.1.0
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: hdu...@tangerine-soft.de
Target Milestone: ---
GCC 6.1. seems to have an issue by using access to storage which __exlicitly__
is casted to an "volatile*" in conjunction with optimization flag –ftree-dse
This issue has been observed with a crosscompiler which is based on the GCC
6.1.0, host: cygwin (32 bit),
for target architecture m68k.
In order to illustrate the issue see 3 Cases (see details below):
1) Code snippet __without__ any explicit type cast to (volatile*).
GCC 6.1.0 with this code produces correct code.
2) Code Snippet __with__ explicit typecast to “volatile short*”.
In conjunction with optimization level -O1/2/3/s this creates wrong code.
The code is wrong that despite the memory location is marked to be volatile*
the
very first access does not happen and seems to be optimized away!
3) Code Snippet __with__ explicit typecast to “volatile short*”.
In conjunction with optimization level -O1/2/3/s but __excluding__
optimization “tree-dse”.
Now GCC 6.1.0 generates right code.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ Example 1) ) Code snippet __without__ any explicit type cast.
+ GCC 6.1.0 with this code produces correct code.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// testmodule.cpp
void fpoke (void *addr, short val, int times, int wait)
{
volatile unsigned short* FLASH = (volatile unsigned short*)0x20000000;
*(FLASH + 0) = 0xAA;
*(FLASH + 1) = 0x55;
*(FLASH + 0) = 0xA0;
}
hdusel@Merlin /cygdrive/w/tmp
$ /opt/gcc-6.1.0-myos/bin/m68k-elf-g++ -O3 testmodule.cpp -c
hdusel@Merlin /cygdrive/w/tmp
$ /opt/gcc-6.1.0-myos/m68k-elf/bin/objdump.exe -d ./testmodule.o
./testmodule.o: file format elf32-m68k
Disassembly of section .text:
00000000 <_Z5fpokePvsii>:
0: 4e56 0000 linkw %fp,#0
4: 207c 2000 0000 moveal #536870912,%a0
a: 4e5e unlk %fp
c: 7055 moveq #85,%d0
e: 30bc 00aa movew #170,%a0@
12: 33c0 2000 0002 movew %d0,20000002 <_Z5fpokePvsii+0x20000002>
18: 30bc 00a0 movew #160,%a0@
1c: 4e75 rts
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ Example 2) Code Snippet __with__ explicit typecast to “volatile short*”.
+ In conjunction with optimization level -O1/2/3/s this creates __wrong code__.
+
+ The __very first__ access to the storage address $20000000 is missing!
+ (see remarks in the code snippet, below)
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// testmodule.cpp
void fpoke (void *addr, short val, int times, int wait)
{
volatile unsigned short* FLASH = (volatile unsigned short*)0x20000000;
*(volatile unsigned short*)(FLASH + 0) = 0xAA; // this line will be optimized
away! Why?
*(volatile unsigned short*)(FLASH + 1) = 0x55;
*(volatile unsigned short*)(FLASH + 0) = 0xA0;
}
hdusel@Merlin /cygdrive/w/tmp
$ /opt/gcc-6.1.0-myos/bin/m68k-elf-g++ -O3 testmodule.cpp -c
hdusel@Merlin /cygdrive/w/tmp
$ /opt/gcc-6.1.0-myos/m68k-elf/bin/objdump.exe -d ./testmodule.o
./testmodule.o: file format elf32-m68k
Disassembly of section .text:
00000000 <_Z5fpokePvsii>:
0: 4e56 0000 linkw %fp,#0
4: 7055 moveq #85,%d0
6: 4e5e unlk %fp
8: 33c0 2000 0002 movew %d0,20000002 <_Z5fpokePvsii+0x20000002>
e: 303c 00a0 movew #160,%d0
12: 33c0 2000 0000 movew %d0,20000000 <_Z5fpokePvsii+0x20000000>
18: 4e75 rts
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ Example 3) Code Snippet __with__ explicit typecast to “volatile short*”.
+ In conjunction with optimization level -O1/2/3/s but __excluding__
optimization “tree-dse”.
+ Now GCC 6.1.0 creates right code.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// testmodule.cpp (The same as on Issue 2)
void fpoke (void *addr, short val, int times, int wait)
{
volatile unsigned short* FLASH = (volatile unsigned short*)0x20000000;
*(volatile unsigned short*)(FLASH + 0) = 0xAA;
*(volatile unsigned short*)(FLASH + 1) = 0x55;
*(volatile unsigned short*)(FLASH + 0) = 0xA0;
}
hdusel@Merlin /cygdrive/w/tmp
$ /opt/gcc-6.1.0-myos/bin/m68k-elf-g++ -O3 -fno-tree-dse testmodule.cpp -c
hdusel@Merlin /cygdrive/w/tmp
$ /opt/gcc-6.1.0-myos/m68k-elf/bin/objdump.exe -d ./testmodule.o
./testmodule.o: file format elf32-m68k
Disassembly of section .text:
00000000 <_Z5fpokePvsii>:
0: 4e56 0000 linkw %fp,#0
4: 207c 2000 0000 moveal #536870912,%a0
a: 4e5e unlk %fp
c: 7055 moveq #85,%d0
e: 30bc 00aa movew #170,%a0@
12: 33c0 2000 0002 movew %d0,20000002 <_Z5fpokePvsii+0x20000002>
18: 30bc 00a0 movew #160,%a0@
1c: 4e75 rts
