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From: "joey.ye at arm dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/55757] Suboptimal interrupt prologue/epilogue for ARMv7-M (Cortex-M3)
Date: Fri, 21 Dec 2012 03:23:00 -0000 [thread overview]
Message-ID: <bug-55757-4-93kt7e98Up@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-55757-4@http.gcc.gnu.org/bugzilla/>
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55757
Joey Ye <joey.ye at arm dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |joey.ye at arm dot com
--- Comment #4 from Joey Ye <joey.ye at arm dot com> 2012-12-21 03:23:07 UTC ---
> An interrupt handler function (void something(void)), but without attribute,
> doing something inside (posts a FreeRTOS semaphore, calls vPortYieldFromISR()
> if it's needed) actually saves a lot of registers on entry:
> 23b4: b507 push {r0, r1, r2, lr}
Pushing of scratch registers can be used to
1. align stack, which Richard has explained
2. allocate stack frame, as a code size optimization of sub sp, #x
Explain with following example:
extern void bar(int *, int *);
void foo()
{
int a, b;
bar(&a, &b);
}
Built with -Os -mcpu=cortex-m3:
push {r0, r1, r2, lr}
Here, pushing of r0 and r1 allocates a 8-byte frame for local variables.
Pushing of r2 is to make sp aligned to 8 bytes together with pushing lr. Values
of r0-r2 pushed to stack don't really matter.
But built with -O2:
push {lr}
sub sp, sp, #12
Former is better on code size, latter wins on performance. Hopefully this
explains everything.
next prev parent reply other threads:[~2012-12-21 3:23 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-12-20 15:04 [Bug rtl-optimization/55757] New: " freddie_chopin at op dot pl
2012-12-20 15:24 ` [Bug rtl-optimization/55757] " freddie_chopin at op dot pl
2012-12-20 16:52 ` rearnsha at gcc dot gnu.org
2012-12-20 17:08 ` freddie_chopin at op dot pl
2012-12-21 3:23 ` joey.ye at arm dot com [this message]
2012-12-21 3:32 ` joey.ye at arm dot com
2012-12-21 7:09 ` freddie_chopin at op dot pl
2013-03-02 15:23 ` web at brolinembedded dot se
2023-11-28 23:10 ` yann at poupet dot eu
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