clang, arch/i386/stand and the calling convention
Joerg Sonnenberger
the last unresolved issues with clang integration are the CMSG* hack and
the size of the boot loader. The former is outside the scope of this
mail, leaving the size of the boot loader. I have some size optimised
versions of the common string routines and with that all of the normal
bootxx images except msdos and ustar fit. The netboot images don't so
far.
The major reason is that LLVM currently doesn't support a push[bwl]
based argument handling like GCC uses for size-optimised code. That
means instead of e.g. using
pushl $3
pushl $2
pushl $1
call foo
it is always using the larger
movl $3, 8(%esp)
movl $2, 4(%esp)
movl $1, (%esp)
call foo
sequence for foo(1,2,3). The second form is normally preferable as it
allows better scheduling.
There are two basic approaches for dealing with this:
(1) Implement the other argument handling style for size optimised code.
This is relatively much work for something not necessarily used often.
(2) Use the more efficient register-based fastcall convention for stand.
This requires changes to the assembler code (but quite a bit is begging
for size optimisation anyway, e.g. libkern), but doesn't involve changes
to the code generator. It is benefical for GCC as well.
Just compiling with -mregparm -mrtd for bootxx gives:
before after
ffsv1 6524 6404
ffsv2 6876 6780
lfsv1 6556 6332
lfsv2 6476 7384
msdos 7512 7372
ustarfs 7612 7372
ext2fs 6916 6856
and allows fitting everything with clang as well. I believe (2) is less
work and helps free some space in general.
Joerg
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