[Bug target/111582] New: [arm-none-eabi-gcc] / suboptimal optimization / bitfield / superfluous stack write

[Bug target/111582] New: [arm-none-eabi-gcc] / suboptimal optimization / bitfield / superfluous stack write

[cptarse-luke at yahoo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111582

            Bug ID: 111582
           Summary: [arm-none-eabi-gcc] / suboptimal optimization /
                    bitfield / superfluous stack write
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cptarse-luke at yahoo dot com
  Target Milestone: ---

When I try to use a struct with a bitfield,
then it happens, that GCC writes to the stack without ever reading it:

> arm-none-eabi-gcc -v
Using built-in specs.
COLLECT_GCC=arm-none-eabi-gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/arm-none-eabi/9.3.0/lto-wrapper
Target: arm-none-eabi
Configured with: ../configure --disable-decimal-float --disable-libffi
--disable-libgomp --disable-libmudflap --disable-libquadmath --disable-libssp
--disable-libstdcxx-pch --disable-libstdc__-v3 --disable-nls --disable-shared
--disable-threads --disable-tls --disable-werror --enable-__cxa_atexit
--enable-c99 --enable-gnu-indirect-function --enable-interwork
--enable-languages=c,c++ --enable-long-long --enable-multilib --enable-plugins
--host= --libdir=/usr/lib --libexecdir=/usr/lib --prefix=/usr
--target=arm-none-eabi --with-gmp --with-gnu-as --with-gnu-ld
--with-headers=/usr/arm-none-eabi/include --with-host-libstdcxx='-static-libgcc
-Wl,-Bstatic,-lstdc++,-Bdynamic -lm' --with-isl --with-libelf --with-mpc
--with-mpfr --with-multilib-list=rmprofile
--with-native-system-header-dir=/include --with-newlib
--with-python-dir=share/gcc-arm-none-eabi --with-sysroot=/usr/arm-none-eabi
--with-system-zlib
Thread model: single
gcc version 9.3.0 (GCC)
# arm-none-eabi-gcc -save-temps -S a.c -O3 -g -mcpu=cortex-m0plus -mthumb -Wall
--specs=nosys.specs -nostdlib -fdata-sections -ffunction-sections
-ffreestanding -Winline
> cat a.i
# 1 "a.c"
# 1 "/tmp//"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "a.c"

typedef unsigned char u8;
typedef unsigned int u32;
extern int fatal();
__attribute__((always_inline)) inline u32 lsb(const u8 l) { return (1U<<l)-1U;
}

typedef struct { u32 a; u32 msk; u32 v; u8 rs:1; u8 aw:7; } Reg;
__attribute__((always_inline)) inline Reg GI(u32 A, u32 N, u32 RS, u8 AW) { Reg
R={A+4*N,lsb(32),0,RS?1:0,AW}; return R; }
__attribute__((always_inline)) inline u32 GS(Reg R) {
   for (u32 i=0, msk=lsb(8); R.aw==1 && msk; i++, msk<<=8)
      if ( !~(R.msk | msk) ) {
         const u8 v = R.v >> (i*8);
         if (R.rs || msk==~R.msk) return (((volatile u8*)R.a)[i] = v) << (i*8);
         else if (R.v==~R.msk) return (((volatile u8*)R.a)[i] |= v) << (i*8);
         return (((volatile u8*)R.a)[i] = (((volatile u8*)R.a)[i] &
(R.msk>>(i*8))) | v) << (i*8);
      }
 return 0;
}
__attribute__((always_inline)) inline Reg GU(Reg R, u32 A, u32 N, u8 o, u8 w,
u32 v) {
   const u32 msk=~(lsb(w)<<o); R.msk&=msk; R.v&=msk;
   R.v |= (v<<o);
   return R;
}
u32 artiSP() { return GS(GU(GI(0xE0000000,42,0,1),0xE0000000,42,17,3,2)); }
> cat a.s
artiSP:
        sub     sp, sp, #16
        mov     r2, sp
        movs    r3, #2
        strb    r3, [r2, #12]
...
        add     sp, sp, #16
        bx      lr

I compile it on a
Intel(R) Pentium(R) Silver J5040 CPU @ 2.00GHz
running Void Linux (kernel: 6.3.13_1)
for a STM32G030.

[Bug target/111582] [arm-none-eabi-gcc] / suboptimal optimization / bitfield / superfluous stack write

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111582

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.0
         Resolution|---                         |FIXED
             Status|UNCONFIRMED                 |RESOLVED
      Known to work|                            |10.4.0, 14.0

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fixed in GCC 10.

[Bug target/111582] [arm-none-eabi-gcc] / suboptimal optimization / bitfield / superfluous stack write

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111582

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> Fixed in GCC 10.

artiSP:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        movs    r2, #14
        @ sp needed
        ldr     r3, .L3
        ldrb    r0, [r3]
        bics    r0, r2
        subs    r2, r2, #10
        orrs    r0, r2
        strb    r0, [r3]
        lsls    r0, r0, #16
        bx      lr
.L4:
        .align  2
.L3:
        .word   -536870742