From: Matt Turner <mattst88@gmail.com>
To: gcc@gcc.gnu.org
Cc: Uros Bizjak <ubizjak@gmail.com>, Richard Henderson <rth@twiddle.net>
Subject: [alpha] Wrong code produced at -Os, -O2, and -O3
Date: Wed, 07 Apr 2010 18:47:00 -0000 [thread overview]
Message-ID: <v2ob4198de61004071138p91fba9aao9fd48d4f91695170@mail.gmail.com> (raw)
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Hi Uros and Richard,
I was rewriting the Alpha sched_find_first_bit implementation for the
Linux Kernel, and in the process I think I've come across a gcc bug.
I rewrote the function using cmov instructions, and wrote a small
program to test its correctness and performance. I wrote the function
initially as an external .S file, and once I was reasonably sure it
was correct, converted it to C function with inline assembly.
Compiling both produce the exact same output, as shown.
<rewritten>:
ldq t0,0(a0)
clr t2
ldq t1,8(a0)
cmoveq t0,0x40,t2
cmoveq t0,t1,t0
cttz t0,t3
addq t3,t2,v0
ret
In my test program, I found that when I executed the rewritten
implementation _before_ the reference implementation that it produced
bogus results. This only happens when using the C/inline asm function.
When compiled with the external .S file, the results are correct.
Attached is a tar.gz with my test code. Compile the test program with
`gcc -O -mcpu=... find.c rewritten.S test.c -o test` with optional
-D__REWRITTEN_INLINE and -D__REWRITTEN_FIRST. At -Os, -O2, or -O3 and
-D__REWRITTEN_INLINE and -D__REWRITTEN_FIRST the program will produce
incorrect results and assert(). At -O0 or -O1 or without one or both
of the -D flags, it will produce correct results. I've tested with
gcc-4.3.4 and gcc-4.4.2.
Thanks. Let me know what I can do to help further.
Matt Turner
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next reply other threads:[~2010-04-07 18:38 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-04-07 18:47 Matt Turner [this message]
2010-04-08 9:06 ` Uros Bizjak
2010-04-08 17:39 ` Matt Turner
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