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Re: GCC -O2 failure for mipsel

To: Andrew Haley <aph@redhat.com>
Subject: Re: GCC -O2 failure for mipsel
From: Fuxin Zhang <fxzhang@ict.ac.cn>
Date: Thu, 01 May 2003 16:41:42 +0800
Cc: MAKE FUN PRANK CALLS <linux-mips@linux-mips.org>, gcc@gcc.gnu.org
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Thanks, -fno-strict-aliasing works.
--The actual code can't be changed: because it is part of spec cpu2000:)

Andrew Haley wrote:

Fuxin Zhang writes:
> Hello,
> I've met a case where mipsel-linux-gcc -O2 fails,for both
> 2.96 and the fresh new 3.2.3. Maybe someone can tell me
> what's wrong.

Your code is incorrect.

> I've reduced the problem to the test case below,compile it
> with mipsel-linux-gcc -O2(FROM H.J.Lu's redhat miniport,all version,
> and 3.2.3 is tested too)
> > > #define PUT_CODE(x,code) ((x)->code = (code))
> union test_union {
> struct test *t;
> int a;
> };
> > struct test {
> unsigned short code;
> union test_union u[1];
> };
> > char memory[2000]; > > struct test *test_alloc(int code)
> {
> struct test *t;
> int length=sizeof(struct test);
> > t = (struct test*)memory;
> length = (sizeof(struct test) - sizeof(union test_union)-1)/sizeof(int);
> for (;length>=0;length--)

This is the errant line:

> ((int*)t)[length] = 0;

You have declared t as a pointer to struct test, but you're using it
as a pointer to int.  If you look at Pointers, Section 6.2.2.3 in ISO
9899-1990 you'll see that this results in undefined behaviour.

-fno-strict-aliasing should generate the code you want, but it's
better to fix your source.  If you want to use a pointer as a
different type, put it in a union.

Andrew.




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