Re: unsigned int to float conversion

[FÖLDY Lajos]

Thanks, so I was missing zero-extension.

As it turned out, the GCC vectorizer was wrong. The bug was fixed in 24 
hours: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40809

regrads,
lajos


On Tue, 28 Jul 2009, sveiki wrote:

> You're compiling here in 64-bit, but keep in mind that your unsigned
> int is 32 bits wide. With the top 32 bits in rax zeroed, rax (signed)
> is equivalent to eax (unsigned), keeping int mind that eax is just the
> least significant 32 bits of rax. The conversion instruction
> (cvtsi2ss) works on 64-bit signed integers (since your operand is a 64-
> bit register).
>
> The 32-bit encoded move, by default zero-extends to 64-bit (according
> to x86).
>
> You might want to check these out: 
> http://www.intel.com/products/processor/manuals/index.htm
>
> Jeremy
>
>
> On Jul 20, 1:15 pm, FÖLDY Lajos <fo...@rmki.kfki.hu> wrote:
> > Hi guys,
> >
> > I have some weird results from my program. The suspect is unsigned int to
> > float conversion. I have created the following simple test:
> >
> > // a.cc
> >
> > void
> > cvt_u4_f4(unsigned int* u4, float* f4)
> > {
> >    *f4=*u4;
> >
> > }
> >
> > Compiling it with g++-4.4 -m64 -O3 -S a.cc creates the following listing:
> >
> >          .file   "a.cc"
> >          .text
> >          .p2align 4,,15
> > .globl _Z9cvt_u4_f4PjPf
> >          .type   _Z9cvt_u4_f4PjPf, @function
> > _Z9cvt_u4_f4PjPf:
> > .LFB0:
> >          .cfi_startproc
> >          .cfi_personality 0x3,__gxx_personality_v0
> >          mov     (%rdi), %eax
> >          cvtsi2ssq       %rax, %xmm0
> >          movss   %xmm0, (%rsi)
> >          ret
> >          .cfi_endproc
> > .LFE0:
> >          .size   _Z9cvt_u4_f4PjPf, .-_Z9cvt_u4_f4PjPf
> >          .ident  "GCC: (GNU) 4.4.0"
> >          .section        .note.GNU-stack,"",@progbits
> >
> > Now, here %eax is filled with the unsigned int value, then %rax is
> > converted to float. It seems to me that the higher 32 bit of %rax is never
> > set, I miss an xor %rax, %rax before mov. Am I right, or I miss something
> > else?
> >
> > thanks,
> > lajos