[Bug c/111500] New: [arm-none-eabi-gcc] / suboptimal optimization

[Bug c/111500] New: [arm-none-eabi-gcc] / suboptimal optimization

[gnu.arne at wgboome dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111500

            Bug ID: 111500
           Summary: [arm-none-eabi-gcc] / suboptimal optimization
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gnu.arne at wgboome dot org
  Target Milestone: ---

Hi!

For my STM32G030K6T I need thumb2 machine code.
arm-none-eabi-gcc can convert my C code quite nicely.

When I inspected the resulting disassembled machine code, I found some funny
artifacts.
Is it worth making&sending minimal source code and/or that corresponding
"preprocessed file *.i*"?
I mean: Is somebody able/willing to do something about it?

Here are the 4 artifacts (when using "-O3 -g"):
1.
a "subs r2, #1" instruction is followed by a "cmp r2, #0 / bne.n" instruction,
although the "subs" already leaves the zero flag properly, doesn't it?
2.
a "b.n" instruction jumps to a "bx lr" instruction, although both have the same
length... i mean: instead of "b.n" it could have written "bx lr"...
3.
a "uxth r2, r2" is followed by a "strh r2, [r3]", although "strh" doesn't look
at the high half-word of r2, does it?
4.
when i use single bits in a struct (e. g.: "struct A { int a:1; int b:7; }"),
gcc reserves 16 bytes on the stack and sometimes writes into this area, without
ever reading from there... when i do it myself with "struct A { int ab; }" and
"if (S.ab&128)..." and "if (S.ab&127)..." then it doesnt touch the stack...

Thx.

Bye.

[Bug target/111500] [arm-none-eabi-gcc] / suboptimal optimization

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111500

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |target
             Status|UNCONFIRMED                 |WAITING
     Ever confirmed|0                           |1
           Keywords|                            |missed-optimization
             Target|                            |arm
   Last reconfirmed|                            |2023-09-20

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Can you attach (compilible) examples code for each issue? Really these should
be filed seperately too.

[Bug target/111500] [arm-none-eabi-gcc] / suboptimal optimization

[gnu.arne at wgboome dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111500

--- Comment #2 from Luke <gnu.arne at wgboome dot org> ---
(In reply to Andrew Pinski from comment #1)
> Can you attach (compilible) examples code for each issue? Really these
> should be filed seperately too.

do u mean, i should file 3 further bug reports?

i try examples for artifact #1 first...
is it right like this?

example code for artifact #1a:
// "cmp" follows the "subs" immediately
__attribute((noinline)) void artiSUBS() {
        for (int i=100; i>0; i--)
200017ec:       2364            movs    r3, #100        ; 0x64
                *(volatile int*)0xE000E014 = i;
200017ee:       4a03            ldr     r2, [pc, #12]   ; (200017fc)
200017f0:       6013            str     r3, [r2, #0]
        for (int i=100; i>0; i--)
200017f2:       3b01            subs    r3, #1
200017f4:       2b00            cmp     r3, #0
200017f6:       d1fb            bne.n   200017f0
}
200017f8:       4770            bx      lr
200017fa:       46c0            nop
200017fc:       e000e014

example code for artifact #1b:
// gcc tends to do the "i--" too early; "ldr" does not touch flags
// maybe because it hopes to get beneficial pipeline effects at loop entry?
__attribute((noinline)) void artiSUBS() {
200017ec:       2364            movs    r3, #100        ; 0x64
        for (int i=100; i>0; i--)
                (void) *(volatile int*)0xE000E014;
200017ee:       4a03            ldr     r2, [pc, #12]   ; (200017fc)
200017f0:       3b01            subs    r3, #1
200017f2:       6811            ldr     r1, [r2, #0]
        for (int i=100; i>0; i--)
200017f4:       2b00            cmp     r3, #0
200017f6:       d1fb            bne.n   200017f0
}
200017f8:       4770            bx      lr
200017fa:       46c0            nop
200017fc:       e000e014

[Bug target/111500] [arm-none-eabi-gcc] / suboptimal optimization

[cptarse-luke at yahoo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111500

--- Comment #3 from Luke <cptarse-luke at yahoo dot com> ---
maybe i should also say the "-v" output?
> 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)

[Bug target/111500] [arm-none-eabi-gcc] / suboptimal optimization

[cptarse-luke at yahoo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111500

--- Comment #4 from Luke <cptarse-luke at yahoo dot com> ---
the a.i file for example #1a is:
# 1 "a.c"
# 1 "/tmp//"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "a.c"
void artiSUBS() {
 for (int i=100; i>0; i--)
  *(volatile int*)0xE000E014 = i;
}

the command-line was:
> 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

and the resulting a.s file contains that subs/cmp sequence...

[Bug target/111500] [arm-none-eabi-gcc] / suboptimal optimization / subs followed by cmp (et alii)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111500
Bug 111500 depends on bug 111580, which changed state.

Bug 111580 Summary: [arm-none-eabi-gcc] / suboptimal optimization / b.n to bx lr
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111580

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

[Bug target/111500] [arm-none-eabi-gcc] / suboptimal optimization / subs followed by cmp (et alii)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111500
Bug 111500 depends on bug 111582, which changed state.

Bug 111582 Summary: [arm-none-eabi-gcc] / suboptimal optimization / bitfield / superfluous stack write
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111582

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED

[Bug target/111500] [arm-none-eabi-gcc] / suboptimal optimization / subs followed by cmp (et alii)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111500

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Luke from comment #4)
> the a.i file for example #1a is:
> # 1 "a.c"
> # 1 "/tmp//"
> # 1 "<built-in>"
> # 1 "<command-line>"
> # 1 "a.c"
> void artiSUBS() {
>  for (int i=100; i>0; i--)
>   *(volatile int*)0xE000E014 = i;
> }
> 
> the command-line was:
> > 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
> 
> and the resulting a.s file contains that subs/cmp sequence...

This is most likely a dup of bug 104773.

[Bug target/111500] [arm-none-eabi-gcc] / suboptimal optimization / subs followed by cmp (et alii)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111500

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #5)
> This is most likely a dup of bug 104773.

Or of bug 3507.

[Bug target/111500] [arm-none-eabi-gcc] / suboptimal optimization / subs followed by cmp (et alii)

[cptarse-luke at yahoo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111500
Bug 111500 depends on bug 111581, which changed state.

Bug 111581 Summary: [arm-none-eabi-gcc] / suboptimal optimization / uxth/sxth
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111581

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
         Resolution|---                         |DUPLICATE

[Bug target/111500] [arm-none-eabi-gcc] / suboptimal optimization / subs followed by cmp (et alii)

[cptarse-luke at yahoo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111500

--- Comment #7 from Luke <cptarse-luke at yahoo dot com> ---
(In reply to Andrew Pinski from comment #6)
> (In reply to Andrew Pinski from comment #5)
> > This is most likely a dup of bug 104773.
> 
> Or of bug 3507.

i concur...
but i do not know which one to choose...
they both look the same to me... somehow...

[Bug target/111500] [arm-none-eabi-gcc] / suboptimal optimization / subs followed by cmp (et alii)

[cptarse-luke at yahoo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111500

Luke <cptarse-luke at yahoo dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #8 from Luke <cptarse-luke at yahoo dot com> ---


*** This bug has been marked as a duplicate of bug 104773 ***