Date: Thu, 5 Sep 1996 07:51:28 +1000 From: Bruce Evans <bde@zeta.org.au> To: bde@zeta.org.au, cvs-all@freefall.freebsd.org, CVS-committers@freefall.freebsd.org, cvs-sys@freefall.freebsd.org, julian@freefall.freebsd.org Subject: Re: cvs commit: src/sys/i386/boot/biosboot Makefile boot.c boot2.S Message-ID: <199609042151.HAA19084@godzilla.zeta.org.au>
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I wrote:
>In the old generated code, howto is kept in a register and flags are
>ORed directly to memory. This takes only 3 bytes per OR for flags
><= 0x80 (using an 8 bit constant). It should take only 3 bytes for
0xffu
>all (single) flags (using an 8 bit constant at an offset), but gcc
>generates poor code for this case (it generates a 32 but constant
>and 6 bytes per OR). ...
The new code copies loadflags to a local varable, so the new generated
code is probably about as mediocre as the old generated code. It can
be better iff the local variable can be allocated in %eax and most of
the constants are < 0x100u. Then the ORing code can take 2 bytes:
orb $const,%al // 2 bytes
For other registers it will be just as large as the indirect method:
orb $const,%dl // 3 bytes
orb $const,0(%anyreg) // 3 bytes
For large constants, best code is larger with a register than indirectly:
orw $bigconst,%ax // 4 bytes (16 bit constant)
orw $bigconst,%dx // 5 bytes (16 bit constant)
orl $bigconst,%eax // 5 bytes (> 16-bit constant)
orl $bigconst,%edx // 6 bytes (> 16-bit constant)
orb $bigconst>>8,1(%anyreg) // 3 bytes
// (0x100u <= bigconst < 0x10000u)
Bruce
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