[Bug c/107415] New: RISCV-gcc: Leaf function compiles as recursive with -O3

[Bug c/107415] New: RISCV-gcc: Leaf function compiles as recursive with -O3

[michael.meier at hexagon dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107415

            Bug ID: 107415
           Summary: RISCV-gcc: Leaf function compiles as recursive with
                    -O3
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: michael.meier at hexagon dot com
  Target Milestone: ---

Created attachment 53776
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53776&action=edit
The -save-temps output as requested

I am using gcc 12.1.0 to compile for RISC-V. Compiling the following leaf
function (it's in the .i file in the attachment) results in gcc generating a
recursive call to memset itself, resulting in an endless loop. The code is a
slightly reduced version function that led me to finding this issue. It does
not make sense as memset, but it triggers the issue.

    void* memset(void* dest, int byte, size_t len)
    {
      if ((((uintptr_t)dest | len) & (sizeof(uintptr_t)-1)) == 0) {
        uintptr_t *d = dest;
        *d = byte;
      } else {
        char *d = dest;
        while (d < (char*)(dest + len))
          *d++ = byte;
      }
      return dest;
    }


First lines of the objdump output of the generated object file:

    00000000 <memset>:
       0:   ff010113                addi    sp,sp,-16
       4:   00812423                sw      s0,8(sp)
       8:   00912223                sw      s1,4(sp)
       c:   00112623                sw      ra,12(sp)
      10:   01010413                addi    s0,sp,16
      14:   00c56733                or      a4,a0,a2
      18:   00377713                andi    a4,a4,3
      1c:   00050493                mv      s1,a0
      20:   02070a63                beqz    a4,54 <.L2>
                            20: R_RISCV_BRANCH      .L2

    00000024 <.LBB2>:
      24:   00c507b3                add     a5,a0,a2
      28:   00f57a63                bgeu    a0,a5,3c <.L4>
                            28: R_RISCV_BRANCH      .L4
      2c:   01859593                slli    a1,a1,0x18

    00000030 <.LVL2>:
      30:   4185d593                srai    a1,a1,0x18
      34:   00000097                auipc   ra,0x0
                            34: R_RISCV_CALL        memset
                            34: R_RISCV_RELAX       *ABS*
      38:   000080e7                jalr    ra # 34 <.LVL2+0x4>

At address 38 inside we find a call to memset. This instruction can be jumped
over if the right conditions on a4 and a0 happen to be true. However running
the function up to the jalr memset does not change any of these conditions.
After calling memset, the PC will arrive will arrive at the jalr memset again,
resulting in an endless loop.


Output of gcc -v:

Using built-in specs.
COLLECT_GCC=/home/memi/Downloads/riscv/bin/riscv64-unknown-elf-gcc
COLLECT_LTO_WRAPPER=/home/memi/Downloads/riscv/bin/../libexec/gcc/riscv64-unknown-elf/12.1.0/lto-wrapper
Target: riscv64-unknown-elf
Configured with: /tmp/rv_gcc/riscv-gnu-toolchain/gcc/configure
--target=riscv64-unknown-elf --prefix=/opt/riscv --disable-shared
--disable-threads --enable-languages=c,c++ --with-pkgversion=g1ea978e3066
--with-system-zlib --enable-tls --with-newlib
--with-sysroot=/opt/riscv/riscv64-unknown-elf
--with-native-system-header-dir=/include --disable-libmudflap --disable-libssp
--disable-libquadmath --disable-libgomp --disable-nls
--disable-tm-clone-registry --src=/tmp/rv_gcc/riscv-gnu-toolchain/gcc
--enable-multilib
--with-multilib-generator='rv32i-ilp32--;rv32ic-ilp32--;rv32im-ilp32--;rv32imc-ilp32--'
--with-abi=lp64d --with-arch=rv64imafdc --with-tune=rocket --with-isa-spec=2.2
'CFLAGS_FOR_TARGET=-Os   -mcmodel=medlow' 'CXXFLAGS_FOR_TARGET=-Os  
-mcmodel=medlow'
Thread model: single
Supported LTO compression algorithms: zlib
gcc version 12.1.0 (g1ea978e3066) 

Complete command line that triggers the problem:

/home/memi/Downloads/riscv/bin/riscv64-unknown-elf-gcc -c own-memset.c  -O3
-DNDEBUG -march=rv32im -mabi=ilp32 -msmall-data-limit=8 -mno-save-restore
-fmessage-length=0 -fsigned-char -ffunction-sections -fdata-sections -g
-fno-omit-frame-pointer -std=gnu11 -MD -MT
softconsole_oriole_main/CMakeFiles/MIV_Project.dir/ISP_PRO.c.obj  -save-temps

The compiler output (on the console):
None

The preprocessed file is attached.

[Bug tree-optimization/107415] RISCV-gcc: Leaf function compiles as recursive with -O3

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107415

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |tree-optimization

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Gcc can detect memset and optimize it to memset. And this is what is happening. 
This is documented too.

[Bug tree-optimization/107415] RISCV-gcc: Leaf function compiles as recursive with -O3

[michael.meier at hexagon dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107415

--- Comment #2 from Michael Meier <michael.meier at hexagon dot com> ---
(In reply to Andrew Pinski from comment #1)
> Gcc can detect memset and optimize it to memset. And this is what is
> happening. 
> This is documented too.

Thanks for your quick reply. I understand what is happening now.

Now that you pointed me in this direction, I found many references to this
behavior only, including that I can use -fno-tree-loop-distribute-patterns to
disable the optimization. However I didn't find mention of the feature in the
official documentation. Can you provide me with a reference, please?

[Bug tree-optimization/107415] RISCV-gcc: Leaf function compiles as recursive with -O3

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107415

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=96628

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup.

*** This bug has been marked as a duplicate of bug 56888 ***