From: Josef Angermeier <sijoange@cip.informatik.uni-erlangen.de>
To: gcc-help@gcc.gnu.org
Subject: gcc's x86 "RET"-machine instruction optimization
Date: Mon, 01 Aug 2005 13:56:00 -0000 [thread overview]
Message-ID: <20050801135646.GC11284@faui00i.informatik.uni-erlangen.de> (raw)
Hello
Im trying to write a script which patches gcc's assembler output (-S
option) in that way, that those functions which are exported (.gobl
asm-directive) return with a near-jump instead of a
near-return-instruction. (Im writing a BIOS for a virtual machine...)
So i fist that it would be an easy task to replace all
"retl"-instructions by "lretw" ones in gcc's asm code. But then now im
facing a problem:
C code like that:
kbd.c:
bios_16_xxxx(struct regs *regs)
{
if (AH == 0x00) {
bios_16_00xx(regs);
} else {
....
}
}
kbd.s
bios_16_xxxx:
.LCFI71:
...
jmp bios_16_00xx
.L105:
...
lretw (PATCHED RETURN)
....
bios_16_00xx:
.LFB59:
...
ret
GCC uses a jmp-instruction instead of a call one, so 'speeds up' execution by
saving the execution of bios_16_xxxx's return instruction (lretw one). This of course
ruins my efforts. Therfore i'd like to know how this optimization is called, and if
there is any gcc option to disable this particular optimization.
Big thanks in advance
josef
next reply other threads:[~2005-08-01 13:56 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-08-01 13:56 Josef Angermeier [this message]
2005-08-01 17:19 ` Ian Lance Taylor
2005-08-01 19:01 ` Josef Angermeier
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20050801135646.GC11284@faui00i.informatik.uni-erlangen.de \
--to=sijoange@cip.informatik.uni-erlangen.de \
--cc=gcc-help@gcc.gnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).