[OpenSmalltalk/opensmalltalk-vm] primitives 54 and 554 for timesTwoPower: answer incorrect result on 64bits macOS VM (#383)

 

This is due to a macOS bug in 64bits libm! (Hard to believe, isn't it?)

I've opened a ticket https://bugreport.apple.com/web/?problemID=48021471

Area:
Something not on this list

Summary: ldexp incorrectly rounds gradual underflow for some specific values.

Steps to Reproduce:
#include <math.h>
#include <stdio.h>
int main() {
int exp=-54; double y=ldexp(11.0,exp); /* y is binary 1.0112^-51 /
double u=ldexp(1.0,-1074); / u is the minimal denormalized IEEE754 double /
double v=ldexp(y,-1023); / v is binary 1.0112^-1074 and should round to u */
printf("u=%g v=%g\n",u,v);
return 0;
}

Expected Results:
v should be rounded to 1.0*2^-1074 (round to nearest, tie to even default rule)
Thus we should have u == v.

Actual Results:
v is rounded upward to binary 10.02^-1074 = 1.02^-1073
output is u=4.94066e-324 v=9.88131e-324

Note 1: this fails for 4 different values of exp -54,-53,+968,+969
(replace exponent -1023 by -1074-3-exp, that is -1023,-1024,-2045,-2046)
Note 2: this did not happen previously with 32bits libm version.
Note 3: this sounds a bit like this bug https://stackoverflow.com/questions/32150888/should-ldexp-round-correctly

Version/Build:
macOS HighSierra 10.13.16

Configuration:
compiled with
clang --version
Apple LLVM version 10.0.0 (clang-1000.11.45.5)
Target: x86_64-apple-darwin17.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
Xcode Version 10.1 (10B61)

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What can we use instead? Platform-specific asm?

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Eliot was working for providing native bit-identical floating point functions for Terf.
So we would use the patched fdlibm once finished.

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Hi Nicolas, Tobias,

On Thu, May 14, 2020 at 12:47 PM Nicolas Cellier <[hidden email]>
wrote:

> Eliot was working for providing native bit-identical floating point
> functions for Terf.
> So we would use the patched fdlibm once finished.
>

It seems to be ready to go. All we do is define
BIT_IDENTICAL_FLOATING_POINT in the makefile. In fact, it might be good to
invert things, and have this be the default unless one defines, e.g.
PLATFORM_SPECIFIC_FLOATING_POINT. What do y'all think?

build.macos64x64$ diff squeak.cog.spur/Makefile croquet.cog.spur/Makefile
5a6,7
> BIT_IDENTICAL_FLOATING_POINT=BIT_IDENTICAL_FLOATING_POINT
>
8a11,17
> # Produce Croquet.app, CroquetAssert.app & CroquetDebug.app
> APPNAME:=Croquet
> APPNAMEDEF:=$(APPNAME)
> APPIDENTIFIER:=org.Croquet.$(APPNAME)
>
> CFLAGS:=-DCROQUET=1
>
11c20
< include ../common/Makefile.app.squeak
---
> include ../common/Makefile.app

_,,,^..^,,,_
best, Eliot

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+1 for PLATFORM_SPECIFIC_FLOATING_POINT

At a glance, I couldn't read the meaning "cross-platform" in BIT_IDENTICAL_FLOATING_POINT :-)

Best,
Marcel

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For what it's worth, I think that if those primitives 54 and 554 fail sometimes, then they can perhaps be added in a Test framework (perhaps those Tests were already written) - the "Run Tests" in Squeak. For example I can see that for some other "float" bug there is such a test : testZeroRaisedToNegativePower
"this is a test related to http://bugs.squeak.org/view.php?id=6781"
(there is also a method testTimesTwoPowerGradualUnderflow in KernelTests-Numbers -> FloatTest -> test - arithmetic.

Ideally I think the Squeak kernel should have some configuration option to be able to use the system provided libmath (or not) so that it's possible to choose which one , when compiling/building the VM.

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I checked the Float-tests (Kernel-Numbers) in Squeak6.0alpha-19793-64bit.image.

I don't immediately see in the Test framework whether there is a Squeak test,
equivalent to the tests for this issue.

Is it perhaps already in the Test framework , maybe I just don't see it ?

David Stes

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