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Date: Thu, 01 May 2003 16:41:42 +0800
From: Fuxin Zhang <fxzhang@ict.ac.cn>
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To: Andrew Haley <aph@redhat.com>
CC: MAKE FUN PRANK CALLS <linux-mips@linux-mips.org>, gcc@gcc.gnu.org
Subject: Re: GCC -O2 failure for mipsel
References: <3EB0B329.9030603@ict.ac.cn> <16048.55936.346808.522687@cuddles.redhat.com>
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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.
>
>
>  
>