[Bug c/60998] New: [MIPS] -O3 produces infinite while() loop. -O2 produces correct code.

["exmortis at yandex dot ru" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60998

            Bug ID: 60998
           Summary: [MIPS] -O3 produces infinite while() loop. -O2
                    produces correct code.
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: exmortis at yandex dot ru

1.      The compiler toolchain was downloaded from CodeSourcer site:
  http://sourcery.mentor.com/public/gnu_toolchain/mips-linux-gnu/mips-2013.11
 -36-mips-linux-gnu-i686-pc-linux-gnu.tar.bz2

  2.      Unpacked to the home dir:
 tar jxf mips-2013.11-36-mips-linux-gnu-i686-pc-linux-gnu.tar.bz2

  3.      Version checking:
 chibisov@lazar:~$ ./mips-2013.11/bin/mips-linux-gnu-gcc -v Using built-in
 specs.
 COLLECT_GCC=./mips-2013.11/bin/mips-linux-gnu-gcc
 COLLECT_LTO_WRAPPER=/home/chibisov/mips-2013.11/bin/../libexec/gcc/mips-linu
 x-gnu/4.8.1/lto-wrapper
 Target: mips-linux-gnu
 Configured with:
 /scratch/cmoore/fall2013-linux-final/src/gcc-4.8-2013.11/configure
 --build=i686-pc-linux-gnu --host=i686-pc-linux-gnu --target=mips-linux-gnu
 --enable-threads --disable-libmudflap --disable-libssp
 --disable-libstdcxx-pch --with-arch-32=mips32r2 --with-arch-64=mips64r2
 --with-float=hard --with-mips-plt --enable-extra-sgxxlite-multilibs
 --with-gnu-as --with-gnu-ld --with-specs='%{save-temps: -fverbose-asm}
 -D__CS_SOURCERYGXX_MAJ__=2013 -D__CS_SOURCERYGXX_MIN__=11
 -D__CS_SOURCERYGXX_REV__=36' --enable-languages=c,c++ --enable-shared
 --enable-lto --enable-symvers=gnu --enable-__cxa_atexit
 --with-pkgversion='Sourcery CodeBench Lite 2013.11-36'
 --with-bugurl=https://sourcery.mentor.com/GNUToolchain/ --disable-nls
 --prefix=/opt/codesourcery
 --with-sysroot=/opt/codesourcery/mips-linux-gnu/libc
 --with-build-sysroot=/scratch/cmoore/fall2013-linux-final/install/opt/codeso
 urcery/mips-linux-gnu/libc
 --with-gmp=/scratch/cmoore/fall2013-linux-final/obj/pkg-2013.11-36-mips-linu
 x-gnu/mips-2013.11-36-mips-linux-gnu.extras/host-libs-i686-pc-linux-gnu/usr
 --with-mpfr=/scratch/cmoore/fall2013-linux-final/obj/pkg-2013.11-36-mips-lin
 ux-gnu/mips-2013.11-36-mips-linux-gnu.extras/host-libs-i686-pc-linux-gnu/usr
 --with-mpc=/scratch/cmoore/fall2013-linux-final/obj/pkg-2013.11-36-mips-linu
 x-gnu/mips-2013.11-36-mips-linux-gnu.extras/host-libs-i686-pc-linux-gnu/usr
 --with-isl=/scratch/cmoore/fall2013-linux-final/obj/pkg-2013.11-36-mips-linu
 x-gnu/mips-2013.11-36-mips-linux-gnu.extras/host-libs-i686-pc-linux-gnu/usr
 --with-cloog=/scratch/cmoore/fall2013-linux-final/obj/pkg-2013.11-36-mips-li
 nux-gnu/mips-2013.11-36-mips-linux-gnu.extras/host-libs-i686-pc-linux-gnu/us
 r
 --with-libelf=/scratch/cmoore/fall2013-linux-final/obj/pkg-2013.11-36-mips-l
 inux-gnu/mips-2013.11-36-mips-linux-gnu.extras/host-libs-i686-pc-linux-gnu/u
 sr --enable-libgomp --disable-libitm --disable-libssp
 --enable-poison-system-directories
 --with-build-time-tools=/scratch/cmoore/fall2013-linux-final/install/opt/cod
 esourcery/mips-linux-gnu/bin
 --with-build-time-tools=/scratch/cmoore/fall2013-linux-final/install/opt/cod
 esourcery/mips-linux-gnu/bin SED=sed Thread model: posix gcc version 4.8.1
 (Sourcery CodeBench Lite 2013.11-36) chibisov@lazar:~$

  4. Minimal test file t.c:

 void *memset(void *s, int c, int count)
 {
      if (count) {
          char *d = s;
          do {
              *d++ = c;
          } while (--count);
      }
      return s;
 }

 5. The compiler is run with recommended keys ("Reporting Bugs" from
 http://gcc.gnu.org/bugs/#detailed)
 chibisov@lazar:~$ ./mips-2013.11/bin/mips-linux-gnu-gcc -Wall -Wextra
 -fno-strict-aliasing -fwrapv -fno-aggressive-loop-optimizations --save-temps
 -O3 -c t.c chibisov@lazar:~$

 No warnings or errors are shown.

 6. The contents of t.i is the following:

 # 1 "t.c"
 # 1 "<command-line>"
 # 1
 "/home/chibisov/mips-2013.11/mips-linux-gnu/libc/usr/include/stdc-predef.h"
 1 3 4 # 30
 "/home/chibisov/mips-2013.11/mips-linux-gnu/libc/usr/include/stdc-predef.h"
 3 4 # 1
 "/home/chibisov/mips-2013.11/mips-linux-gnu/libc/usr/include/bits/predefs.h"
 1 3 4 # 31
 "/home/chibisov/mips-2013.11/mips-linux-gnu/libc/usr/include/stdc-predef.h"
 2 3 4 # 1 "<command-line>" 2 # 1 "t.c"

 void *memset(void *s, int c, int count)
 {
      if (count) {
  char *d = s;

  do {
      *d++ = c;
  } while (--count);
      }
      return s;
 }

 7. The contents of t.s is the following:

        .file  1 "t.c"
        .section .mdebug.abi32
        .previous
        .nan    legacy
        .gnu_attribute 4, 1
        .abicalls
        .option pic0

  # -G value = 0, Arch = mips32r2, ISA = 33
  # GNU C (Sourcery CodeBench Lite 2013.11-36) version 4.8.1 (mips-linux-gnu)
  #      compiled by GNU C version 4.7.2, GMP version 5.0.5, MPFR version
 3.1.1-p2, MPC version 1.0.1
  # GGC heuristics: --param ggc-min-expand=100 --param
 ggc-min-heapsize=131072
  # options passed:  -fpreprocessed t.i -meb -march=mips32r2 -mhard-float
  # -mllsc -mplt -mno-shared -mabi=32 -O3 -Wall -Wextra -fno-strict-aliasing
  # -fwrapv -fno-aggressive-loop-optimizations -fverbose-asm
  # options enabled:  -fauto-inc-dec -fbranch-count-reg -fcaller-saves
  # -fcombine-stack-adjustments -fcommon -fcompare-elim -fcprop-registers
  # -fcrossjumping -fcse-follow-jumps -fdefer-pop -fdelayed-branch
  # -fdelete-null-pointer-checks -fdevirtualize -fdwarf2-cfi-asm
  # -fearly-inlining -feliminate-unused-debug-types
  # -fexpensive-optimizations -fforward-propagate -ffunction-cse -fgcse
  # -fgcse-after-reload -fgcse-lm -fgnu-runtime -fguess-branch-probability
  # -fhoist-adjacent-loads -fident -fif-conversion -fif-conversion2
  # -findirect-inlining -finline -finline-atomics -finline-functions
  # -finline-functions-called-once -finline-small-functions -fipa-cp
  # -fipa-cp-clone -fipa-profile -fipa-pure-const -fipa-reference -fipa-sra
  # -fira-hoist-pressure -fira-share-save-slots -fira-share-spill-slots
  # -fivopts -fkeep-static-consts -fleading-underscore -fmath-errno
  # -fmerge-constants -fmerge-debug-strings -fmove-loop-invariants
  # -fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls
  # -foptimize-strlen -fpartial-inlining -fpcc-struct-return -fpeephole
  # -fpeephole2 -fpredictive-commoning -fprefetch-loop-arrays -fregmove
  # -freorder-blocks -freorder-functions -frerun-cse-after-loop
  # -fsched-critical-path-heuristic -fsched-dep-count-heuristic
  # -fsched-group-heuristic -fsched-interblock -fsched-last-insn-heuristic
  # -fsched-rank-heuristic -fsched-spec -fsched-spec-insn-heuristic
  # -fsched-stalled-insns-dep -fschedule-insns -fschedule-insns2
  # -fshow-column -fshrink-wrap -fsigned-zeros -fsplit-ivs-in-unroller
  # -fsplit-wide-types -fstrict-overflow -fstrict-volatile-bitfields
  # -fsync-libcalls -fthread-jumps -ftoplevel-reorder -ftrapping-math
  # -ftree-bit-ccp -ftree-builtin-call-dce -ftree-ccp -ftree-ch
  # -ftree-coalesce-vars -ftree-copy-prop -ftree-copyrename -ftree-cselim
  # -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre
  # -ftree-loop-distribute-patterns -ftree-loop-if-convert -ftree-loop-im
  # -ftree-loop-ivcanon -ftree-loop-optimize -ftree-parallelize-loops=
  # -ftree-partial-pre -ftree-phiprop -ftree-pre -ftree-pta -ftree-reassoc
  # -ftree-scev-cprop -ftree-sink -ftree-slp-vectorize -ftree-slsr
  # -ftree-sra -ftree-switch-conversion -ftree-tail-merge -ftree-ter
  # -ftree-vect-loop-version -ftree-vectorize -ftree-vrp -funit-at-a-time
  # -funswitch-loops -fuse-caller-save -fvect-cost-model -fverbose-asm
  # -fwrapv -fzero-initialized-in-bss -mabicalls -mcheck-zero-division
  # -mdivide-traps -mdouble-float -meb -mexplicit-relocs -mextern-sdata
  # -mfp-exceptions -mfp32 -mfused-madd -mglibc -mgp32 -mgpopt -mhard-float
  # -mimadd -mllsc -mlocal-sdata -mlong32 -mno-mdmx -mno-mips16 -mno-mips3d
  # -mplt -mrelax-pic-calls -msplit-addresses -mvr4130-align

        .text
        .align  2
        .globl  memset
        .set    nomips16
        .set    nomicromips
        .ent    memset

        .type  memset, @function
 memset:
        .frame  $sp,32,$31              # vars= 0, regs= 2/0, args= 16, gp=
 8
        .mask  0x80010000,-4
        .fmask  0x00000000,0
         .set    noreorder
        .set    nomacro
        addiu  $sp,$sp,-32      #,,
        sw      $16,24($sp)      #,
        sw      $31,28($sp)      #,
        beq    $6,$0,$L4        #, count,,
        move    $16,$4  # s, s

        jal    memset  #
        seb    $5,$5    #, c

 $L4:
        lw      $31,28($sp)      #,
        move    $2,$16  #, s
        lw      $16,24($sp)      #,
        j      $31      #
        addiu  $sp,$sp,32      #,,

        .set    macro
        .set    reorder
        .end    memset
        .size  memset, .-memset
        .ident  "GCC: (Sourcery CodeBench Lite 2013.11-36) 4.8.1"

 8. The disassembly listing of t.o:

 chibisov@lazar:~$ ./mips-2013.11/bin/mips-linux-gnu-objdump -d t.o

 t.o:    file format elf32-tradbigmips

 Disassembly of section .text:

 00000000 <memset>:
    0:  27bdffe0        addiu  sp,sp,-32
    4:  afb00018        sw      s0,24(sp)
    8:  afbf001c        sw      ra,28(sp)
    c:  10c00003        beqz    a2,1c <memset+0x1c>
  10:  00808021        move    s0,a0
  14:  0c000000        jal    0 <memset>
  18:  7c052c20        seb    a1,a1
  1c:  8fbf001c        lw     ra,28(sp)
  20:  02001021        move    v0,s0
  24:  8fb00018        lw      s0,24(sp)
  28:  03e00008        jr      ra
  2c:  27bd0020        addiu  sp,sp,32

 chibisov@lazar:~$

 The error is at offset 0x14 - jmp to beginning of the function memset:

 jal    0 <memset>

 9. If "nop" is placed inside the loop, the generated code magically becomes
 correct:

 The contents of t.c:
 void *memset(void *s,int c,size_t count) {
     if (count) {
         char *d = s;
          do {
             *d++ = c;
              __asm__ __volatile__ (".word 0x25\n" : : );
         } while (--count);
     }
     return s;
 }

  Disassembly of t.o:

 00000000 <memset>:
   0: 10c00009 beqz a2,28 <memset+0x28>
   4: 00801021 move v0,a0
   8: 30a500ff andi a1,a1,0xff
   c: 00863021 addu a2,a0,a2
   10: 00801821 move v1,a0
   14: 24630001 addiu v1,v1,1
   18: a065ffff sb  a1,-1(v1)
   1c: 00000025 move zero,zero
   20: 1466fffd bne  v1,a2,18 <memset+0x18>
   24: 24630001 addiu v1,v1,1
   28: 03e00008 jr  ra
   2c: 00000000 nop

 10. If t.c is compiled with -O2 the generated code is correct:

 chibisov@lazar:~$ ./mips-2013.11/bin/mips-linux-gnu-gcc -Wall -Wextra
 -fno-strict-aliasing -fwrapv -fno-aggressive-loop-optimizations --save-temps
 -O2 -c t.c chibisov@lazar:~$ ./mips-2013.11/bin/mips-linux-gnu-objdump -d
 t.o

 t.o:    file format elf32-tradbigmips

 Disassembly of section .text:

 00000000 <memset>:
    0:  10c00007        beqz    a2,20 <memset+0x20>
    4:  00801021        move    v0,a0
    8:  7c052c20        seb    a1,a1
    c:  00863021        addu    a2,a0,a2
  10:  00801821        move    v1,a0
  14:  24630001        addiu  v1,v1,1
  18:  1466fffe        bne    v1,a2,14 <memset+0x14>
  1c:  a065ffff        sb      a1,-1(v1)
  20:  03e00008        jr      ra
  24:  00000000        nop