[Vm-dev] Aarch64 number bug

[Tobias Pape]

Hi


> On 21. Jun 2021, at 12:15, Bruce O'Neel <bruce.oneel at pckswarms.ch> wrote:
> 
> Hi,
> 
> I have now gone back and checked this carefully.  The git repository at the commit before this one produces the right answer for 2 raisedTo: 64 on Aarch64.  It produces the wrong answer, 0, when this commit is included.
> 
> So it's this commit.  I'll look at the code to see if I can see why the code is bad.
> 
> BTW, how would one find the two referenced packages?

https://source.squeak.org/VMMaker/ has all the packages. (or http://source.squeak.org/VMMaker.html)

	The diff for the first one (https://source.squeak.org/VMMaker/VMMaker.oscog-eem.2799.diff) ist truncated,
	while the diff-changeset is ok: http://source.squeak.org/VMMaker/changes/VMMaker.oscog-eem.2799(2798).cs
	(you see all changed methods, but not the actual diff)

But this only adds the JumpMulOverflow / JumpNoMulOverflow names/class-vars.

The implementation diff is found at
	http://www.squeaksource.com/ClosedVMMaker/ClosedVMMaker-eem.98.diff

I would suggest that our bug does not concern Jumps, but rather only Mul, so we have to start looking
at the differences for concretizeMulOverflowRRR ...

From what I see, tho, is that the MUL and the SMULH instruction order has been reversed.

(Also, the arm blog uses an different interesting method of detecting overflow,
by missusing the argument shitfter of CMP: https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/detecting-overflow-from-mul
Although this is written for older arm, it seems to be applicable to the 64-variant too)

Best regards
	-Tobias



> cheers
> 
> bruce
> 
> On 2021-06-20T22:44:36.000+02:00, Bruce O'Neel <bruce.oneel at pckswarms.ch> wrote:
> Playing with Git bisect gets us to the commit below as the first bad one.
> 
> I started the git bisect with the two commits below.  The first one is bad, the second is the good one.
> 
> a783502b249c4a4fedc88b6e07837d405feab144 - zero9 - builds, zero error.
> 
> 9f73148b8da4bc00278b83faa8da6b1c418fa54f - zero10 - builds works
> 
> Now this is not 100% guaranteed because one of the commits that git bisect chose had a build error so I had to mark that one as bad.  But the commit found does sound possible.
> 
> f632ee2888014ee88330ee994e13c9c609b57b5f is the first bad commit
> commit f632ee2888014ee88330ee994e13c9c609b57b5f
> Author: Eliot Miranda <eliot.miranda at gmail.com>
> Date:   Wed Sep 2 10:45:27 2020 -0700
> 
>     CogVM source as per VMMaker.oscog-eem.2799/ClosedVMMaker-eem.98
>     
>     Ha!  I am *STUPID*.
>     Integer overflow is not only determined by the upper 64-bits of a 64x64=>128 bit
>     multiply being either all zero or all ones (0 or -1), but by the upper 64-bits
>     being an extension of the most significant bit of the lower 64 bits!!
> 
> :040000 040000 ffb8fbcd8ab5e2ca1936429b2daaade62909c178 a5ac89d8b1d4980c5329835cdd3d4a2387b1fac5 M      nsspur64src
> :040000 040000 e606dac14ee1db4ac59b0949bb03cd8e657d7aa7 be666051cd1d52d0055e319432b66a0fba61063e M      spur64src
> :040000 040000 1dea4faf99821c60e2c2461076bfe8b99d4dea9b afbef904e8cbc7e4b80e1606420dd6293e12f3e9 M      spurlowcode64src
> :040000 040000 f66c681004806d5880af7c0a3115038f4f2f3361 0e8d76bedcf22b6ad7497ea2e3dc44f3c36c2f3a M      spursista64src
> 
> On 2021-06-20T21:48:49.000+02:00, Craig Latta <craig at blackpagedigital.com> wrote:
> Hi Ken--
> 
> I brought up a development environment on a Raspberry Pi 4.
> 
> > Unfortunately, my attempt to build a VM simulator on aarch64 fails due
> > to a divide by zero bug.
> >
> > Perhaps because
> > 16r8000000000000000 printStringHex. ==> '0'
> 
> update-eem-463.mcm has a postload action which turns on Sista, 
> which recompiles the system, which tries to recompile 
> FloatArrayTest>>testFloatArrayPluginPrimitiveAtPut, which exercises this 
> very bug during scanning, when trying to create the float 1e-127.
> 
> For now I've rewritten testFloatArrayPluginPrimitiveAtPut and 
> testFloatArrayPluginPrimitiveAt on my system to do nothing. When 
> building GdbARMv8Plugin, there's some other kind of 
> header-include-ordering mayhem, similar to the problem with Linux 
> features.h when building the VM. (At least two levels of it, between 
> bfd.h and the aarch64 simulator's config.h, and config.h and other 
> system headers.) I expect no one has successfully built GdbARMv8Plugin 
> on anything but a M1 macOS machine so far.
> 
> 
> -C