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Re: [ACPI] Why does gcc suck at switch()?


Hi!

> Why does gcc generate worse code for switch() statements than for
> multiple-if?
> 
> $ gcc --version
> gcc (GCC) 3.3.2 20030908 (Debian prerelease)
> 
> I would expect a compiler to produce the same code for these cases.
> Instead, switch is worse than the multiple-if:
> 
> -       switch (event) {
> -       case ACPI_THERMAL_NOTIFY_TEMPERATURE:
> +       if (event == ACPI_THERMAL_NOTIFY_TEMPERATURE) {
>                 acpi_thermal_check(tz);
> -               break;
> -       case ACPI_THERMAL_NOTIFY_THRESHOLDS:
> +       } else if (event == ACPI_THERMAL_NOTIFY_THRESHOLDS) {
>                 acpi_thermal_get_trip_points(tz);
>                 acpi_thermal_check(tz);
>                 acpi_bus_generate_event(device, event, 0);
> -               break;
> -       case ACPI_THERMAL_NOTIFY_DEVICES:
> +       } else if (event == ACPI_THERMAL_NOTIFY_DEVICES) {
>                 if (tz->flags.devices)
>                         acpi_thermal_get_devices(tz);
>                 acpi_bus_generate_event(device, event, 0);
> -               break;
> -       default:
> +       } else {
>                 ACPI_DEBUG_PRINT((ACPI_DB_INFO,
>                         "Unsupported event [0x%x]\n", event));
> -               break;
>         }
> 
> $ size *.o
>    text    data     bss     dec     hex filename
>    5996     704      16    6716    1a3c thermal-multiple-if.o
>    6005     704      16    6725    1a45 thermal-switch.o
> 
> Here's the asm diff between the two versions:
> 
> -       cmpl    $129, %esi
> -       je      .L597
> -       cmpl    $129, %esi
> -       ja      .L602
> -       addl    $-128, %esi
> -       je      .L596
> -       jmp     .L592
> -.L602:
> -       cmpl    $130, %esi
> -       je      .L598
> -       jmp     .L592
> -.L596:
> +       cmpl    $128, %esi
> +       jne     .L595
>  ...
> -.L597:
> +.L595:
> +       cmpl    $129, %esi
> +       jne     .L597
>  ...
> -.L598:
> +.L597:
> +       cmpl    $130, %esi
> +       jne     .L592
> 
> The other diffs are just label numbers changing.  Given that gcc was
> instructed to optimise for space (full command line:
> 
> gcc -Wp,-MD,drivers/acpi/.thermal.o.d -nostdinc -iwithprefix include \
> 	-D__KERNEL__ -Iinclude  -D__KERNEL__ -Iinclude  -Wall \
> 	-Wstrict-prototypes -Wno-trigraphs -O2 -fno-strict-aliasing \
> 	-fno-common -pipe -mpreferred-stack-boundary=2 -march=i686 \
> 	-Iinclude/asm-i386/mach-default -fomit-frame-pointer  -Os   \
> 	-DKBUILD_BASENAME=thermal -DKBUILD_MODNAME=thermal -c \
> 	-o drivers/acpi/thermal.o drivers/acpi/thermal.c
> 
> I think it should choose the more space-efficient approach.

Okay, but please don't change kernel code to workaround compiler
problem. This should probably be sent to gcc mailing list... Ahha, it
is. Not sure if it needs to be cc-ed to acpi at all.

								Pavel

-- 
When do you have a heart between your knees?
[Johanka's followup: and *two* hearts?]


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