[Bug c++/88562] New: Incorrect pointer incrementing on ST-SH4

[zavadovsky.yan at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88562

            Bug ID: 88562
           Summary: Incorrect pointer incrementing on ST-SH4
           Product: gcc
           Version: 8.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zavadovsky.yan at gmail dot com
  Target Milestone: ---

Created attachment 45268
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=45268&action=edit
code to trigger the bug

Hello.

I have got some piece of source code which incorrectly works after optimized
compilation for ST-SH4 architecture.
But it works good with some other CPUs - checked on x86-64, armv8 and mips.
And works good with some optimizations off.

Details:

Here is the minimal piece of code to trigger this bug:
<code begin>
unsigned Read32(const unsigned char* ptr);

const unsigned char* ReadQm(const unsigned char* ptr, const unsigned char* end)
{
        const unsigned char* ret = 0;

        while(ptr < end)
        {
                unsigned char tag = *ptr++;
                unsigned len = Read32(ptr);
                ptr += 4;
                if (!tag || !len)
                        break;
                if (ptr + len > end)
                        break;
                ret = ptr;
                ptr += len;
        }

        return ret;
}
<code end>

The error is in 'ptr += 4;' statement.
It is compiled like 'ptr += 5;'.
This leads to non-working assembler opcodes.

But some changes to this code will prevent this error.
I've checked these variants:
- replace 'return ret;' with 'return ptr;'
- add line 'AnyFuncWhichTakesPointerAsArgument(ptr);' before 'return ret;'
- compile without some optimizations


How to compile and get the bug: 'sh4-unknown-linux-gnu-g++ -Os -S -c
ReadQm.cpp'

Assembler output with some comments:
<asm begin>
        // r4 == ptr
        cmp/hs  r11,r4
        bt/s    .L8
        mov     r10,r0
        mov     r4,r8 // r8 = ptr
        mov.b   @r8+,r9 // r9 = tag, r8 = (ptr + 1)
        mov     r8,r4 
        jsr     @r12 // call Read32(ptr + 1)
        add     #5,r8 // r8 = ((ptr + 1) + 5) HERE IS ERROR, must be '+=4'
<asm end>


How to compile and DON'T get the bug: 'sh4-unknown-linux-gnu-g++ -Os
-fno-expensive-optimizations -S -c ReadQm.cpp'

Assembler output with some comments:
<asm begin>
        // r8 == ptr  
        mov     r8,r4 // r4 = ptr
        jsr     @r12 // call Read32(ptr + 1), note: this line is executed after
next because of SH4's 'delayed slot' stuff
        mov.b   @r4+,r9 // r9 = tag, r4 = (ptr + 1)
        tst     r9,r9
        mov     r8,r1 // r1 = ptr
        bt/s    .L5
        add     #5,r1 // r1 = ((ptr + 1) + 4) ALL GOOD HERE
<asm end>


GCC info:
yan@ws-zavadovskiy:~/000_sh4_qmreader$ ./820/bin/sh4-unknown-linux-gnu-g++ -v
Using built-in specs.
COLLECT_GCC=./820/bin/sh4-unknown-linux-gnu-g++
COLLECT_LTO_WRAPPER=/yan/000_sh4_qmreader/820/bin/../libexec/gcc/sh4-unknown-linux-gnu/8.2.0/lto-wrapper
Target: sh4-unknown-linux-gnu
Configured with:
/yan/000_sh4_gcc/ct-ng/.build/sh4-unknown-linux-gnu/src/gcc/configure
--build=x86_64-build_pc-linux-gnu --host=x86_64-build_pc-linux-gnu
--target=sh4-unknown-linux-gnu
--prefix=/yan/000_sh4_gcc/ct-ng/../sh4-unknown-linux-gnu
--with-sysroot=/yan/000_sh4_gcc/sh4-unknown-linux-gnu/sh4-unknown-linux-gnu/sysroot
--enable-languages=c,c++ --with-pkgversion='crosstool-NG 1.23.0.580-eb72b4e'
--enable-__cxa_atexit --disable-libmudflap --disable-libgomp --disable-libssp
--disable-libquadmath --disable-libquadmath-support --disable-libsanitizer
--disable-libmpx
--with-gmp=/yan/000_sh4_gcc/ct-ng/.build/sh4-unknown-linux-gnu/buildtools
--with-mpfr=/yan/000_sh4_gcc/ct-ng/.build/sh4-unknown-linux-gnu/buildtools
--with-mpc=/yan/000_sh4_gcc/ct-ng/.build/sh4-unknown-linux-gnu/buildtools
--with-isl=/yan/000_sh4_gcc/ct-ng/.build/sh4-unknown-linux-gnu/buildtools
--disable-lto --with-host-libstdcxx='-static-libgcc -Wl,-Bstatic,-lstdc++ -lm'
--enable-threads=posix --enable-target-optspace --disable-plugin --disable-nls
--enable-multiarch
--with-local-prefix=/yan/000_sh4_gcc/sh4-unknown-linux-gnu/sh4-unknown-linux-gnu/sysroot
--enable-long-long
Thread model: posix
gcc version 8.2.0 (crosstool-NG 1.23.0.580-eb72b4e) 


I've additionally checked some GCC versions:
GCC 4.6 and 4.7 on st-sh4, x86-64, armv7, mips - no bug
GCC 4.9.4 on st-sh4 - no bug
GCC 5.4.0 on st-sh4 - has bug
GCC 6.3.0 on armv8, mips - no bug
GCC 6.3.0 on st-sh4 - has bug
GCC 6.5.0 on st-sh4 - has bug
GCC 7.4.0 on st-sh4 - has bug