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Date: Fri, 16 Feb 2007 09:38:29 -0500
From: "Lev Bishop" <lev DOT bishop AT gmail DOT com>
To: "Andrew Makhorin" <mao AT gnu DOT org>, cygwin AT cygwin DOT com
Subject: Re: strange bug in gettimeofday function
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On 2/16/07, Andrew Makhorin wrote:

> Nevertheless, you agree that if t0 > t1 then t0 - t1 cannot be exact
> zero in *any* floating-point model, don't you? Even if optimization is
> used, the compiler must not arbitrarily change the precision of the
> same floating-point variable.

The -ffloat-store compiler option is available for those programs who
are concerned with such details. This is your code compiled -O2 and
-f{no-,}float-store. The difference is one extra load instruction.

$ diff -u f-floatstore.s f-nofloatstore.s
--- f-floatstore.s      2007-02-16 04:26:19.117875000 -0500
+++ f-nofloatstore.s    2007-02-16 04:26:03.399125000 -0500
@@ -321,11 +321,10 @@
        fxch    %st(1)
 .stabn 68,0,12,LM12-_main
 LM12:
-       fstpl   -16(%ebp)
+       fstl    -16(%ebp)
+       fxch    %st(1)
 .stabn 68,0,13,LM13-_main
 LM13:
-       fldl    -16(%ebp)
-       fxch    %st(1)
        fucompp
        fnstsw  %ax
        sahf

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