Re: [Qemu-devel] [qemu-s390x] [PATCH] pc-bios/s390-ccw: Optimize the s390-ccw bios for size

[Thomas Huth]

On 14.06.2018 12:55, Christian Borntraeger wrote:
> 
> On 06/14/2018 10:48 AM, Thomas Huth wrote:
>> Since we're storing the firmware blobs in the QEMU git repository, it
>> would be nice if the blobs would be a little bit smaller. By using -Os
>> and -fno-asynchronous-unwind-tables the size of the s390-ccw.img can be
>> decreased by ca. 4kB, and the size of s390-netboot.img even by ca. 45kB.
>> The change of speed of the firmware should not really matter compared to
>> a binary that has been compiled with -O2 since the firmware still starts
>> very fast.
> 
> 
>>
>> Signed-off-by: Thomas Huth <address@hidden>
>> ---
>>  pc-bios/s390-ccw/Makefile | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/pc-bios/s390-ccw/Makefile b/pc-bios/s390-ccw/Makefile
>> index 439e3cc..f22a5ca 100644
>> --- a/pc-bios/s390-ccw/Makefile
>> +++ b/pc-bios/s390-ccw/Makefile
>> @@ -13,6 +13,7 @@ OBJECTS = start.o main.o bootmap.o jump2ipl.o sclp.o 
>> menu.o \
>>        virtio.o virtio-scsi.o virtio-blkdev.o libc.o
>>  
>>  QEMU_CFLAGS := $(filter -W%, $(QEMU_CFLAGS))
>> +QEMU_CFLAGS += -Os -fno-asynchronous-unwind-tables
> 
> Hmm, it looks like that the O2 comes after Os for me (which would override 
> Os).
> 
> cc -iquote /home/cborntra/REPOS/qemu/build/. -iquote . -iquote 
> /home/cborntra/REPOS/qemu/tcg -iquote /home/cborntra/REPOS/qemu/tcg/s390 
> -I/home/cborntra/REPOS/qemu/linux-headers 
> -I/home/cborntra/REPOS/qemu/build/linux-headers -iquote . -iquote 
> /home/cborntra/REPOS/qemu -iquote /home/cborntra/REPOS/qemu/accel/tcg -iquote 
> /home/cborntra/REPOS/qemu/include -Werror -Wstrict-prototypes 
> -Wredundant-decls -Wall -Wundef -Wwrite-strings -Wmissing-prototypes 
> -Wexpansion-to-defined -Wendif-labels -Wno-shift-negative-value 
> -Wno-missing-include-dirs -Wempty-body -Wnested-externs -Wformat-security 
> -Wformat-y2k -Winit-self -Wignored-qualifiers -Wold-style-declaration 
> -Wold-style-definition -Wtype-limits -Os -fno-asynchronous-unwind-tables 
> -ffreestanding -fno-delete-null-pointer-checks -msoft-float -march=z900 -fPIE 
> -fno-strict-aliasing   -fno-stack-protector -MMD -MP -MT libc.o -MF ./libc.d 
> -O2 -g  -c -o libc.o /home/cborntra/REPOS/qemu/pc-bios/s390-ccw/libc.c
> 
> so the effect is only because of the -fno-asynchronous-unwind-tables.

Ugh, you're right - at least for the objects for s390-ccw.img. Seems
like the -O2 comes from "CFLAGS" and not from "QEMU_CFLAGS" here, so the
flags should be added to the CFLAGS variable instead.

Actually, I was only looking at the object files from s390-netboot.img,
and the libc files from SLOF get *only* compiled with QEMU_CFLAGS there
(see LIBC_CFLAGS in netboot.mak). So the huge part of the
s390-netboot.img binary got compiled without optimizations so far!
That's why I was seeing such a big improvement there! Wow, I wonder why
we did not notice this before... Anyway, I'll send a v2 to fix these issues.

 Thomas