From: MikeW <mw_phil@yahoo.co.uk>
To: gcc-help@gcc.gnu.org
Subject: gcc optimises out test of value in register-only loop
Date: Thu, 13 Oct 2011 11:35:00 -0000 [thread overview]
Message-ID: <loom.20111013T124012-691@post.gmane.org> (raw)
sh4-linux-gcc (GCC) 4.2.4 [unfortunately the version is tied to the
kernel build. ]
In some kernel code where RAM is unavailable due to manipulation of the MMU,
I wanted to place some 'got here' stops in the code so I could ^C break in gdb,
reset a register value and allow execution to continue.
Accordingly I tried:
volatile register int stop_loop __asm("r5")__;
...
stop_loop = 0x1234;
(disable MMU)
while (stop_loop != 0);
...
which seemed to generate code that checks the value of r5 only once:
xxxx08: mov r5,r1
xxxx0a: tst r1,r1
xxxx0c: bf xxxx0c ;r5 never tested again!!
xxxx0e: (unrelated code)
I also tried
while ((volatile)stop_loop != 0);
and
while ((volatile)(stop_loop != 0));
which both gave the original asm code as above.
So, in short, is there any way to persuade gcc to reload r5 - which could
in other non-debug situations be a global register variable updated
in an ISR, for example.
Or perhaps this is 'fixed' in a later gcc ...
next reply other threads:[~2011-10-13 11:35 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-10-13 11:35 MikeW [this message]
2011-10-13 11:38 ` Andrew Haley
2011-10-13 11:56 ` MikeW
2011-10-13 11:58 ` Thomas Martitz
2011-10-13 12:01 ` Andrew Haley
2011-10-13 12:14 ` MikeW
2011-10-13 12:18 ` Andrew Haley
2011-10-13 12:05 ` MikeW
2011-10-14 4:41 ` Ian Lance Taylor
2011-10-14 9:07 ` Andrew Haley
2011-10-15 3:20 ` Ian Lance Taylor
2011-10-17 8:30 ` Andrew Haley
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