Ian Lynagh wrote:
>
> Hi Brad,
>
> On Tue, Nov 20, 2007 at 09:50:02PM +1000, Brad Clow wrote:
> >
> > $ ./test
> > 23
> > 24
>
> I can't reproduce this. Can you please tell us what platform you are on
> (e.g. x86_64 Linux) and what gcc --version says?
I see a bug that only affects x86_32.
The native code generator (compiler/nativeGen/PprMach.hs) generates a
simple fistp instruction for converting floats or doubles to int. with
the usual default rounding mode (round) that means numbers between n+1/2
and n are rounded up instead of truncated. (relevant Code:
pprInstr g@(GDTOI src dst)
= pprG g (hcat [gtab, text "subl $4, %esp ; ",
gpush src 0, gsemi, text "fistpl 0(%esp) ; popl ",
pprReg I32 dst])
)
What's missing is the FPU control word manipulation to set the
rounding mode appropriately, say
pprinstr g@(GDTOI src dst)
= pprG g (hcat [gtab, gpush src 0,
text " ; subl $4, %esp ; fstcw 0(%esp) ; movl $0xC00, ",
reg, text " ; orl 0(%esp), ", reg,
text " ; pushl ", reg,
text " ; fldcw 0(%esp) ; fistpl 0(%esp) ; popl ", reg,
text " ; addl $4, %esp"])
where
reg = pprReg I32 dst
For reference, see
http://www.eglibc.org/cgi-bin/viewcvs.cgi/fsf/glibc-2_3-branch/libc/sysdeps/i386/fpu/s_truncf.S?rev=10&sortby=date&view=markup
HTH,
Bertram
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