[cfe-dev] Possible bug when using -O2/-O3 in clang 13 for ARMv7

[Mikhail R. Gadelha via cfe-dev]

Hi,

Em seg., 13 de dez. de 2021 às 06:27, David Spickett <
david.spickett at linaro.org> escreveu:

> FWIW I think opening an issue with what you've got would be fine.
>
>
Cool, I'll try to add the information here and open the github issue.


> > when I build JSC using either -O2 or -O3, I get random garbage when
> querying for the "Infinity" constant from javascript
>
> Can you elaborate on what JSC is and how you do the query? Is it something
> like:
> * build an interpreter
> * interpret javascript code that prints infinity
> * check for expected value
>

Right, so to build JSC, you need to get WebKit from
https://github.com/WebKit/WebKit (shallow clone is a friend here), and run:

$ ./Tools/Scripts/build-jsc --Release --jsc-only
'--cmakeargs=-DCMAKE_CXX_COMPILER=<path-to-clang-13>/bin/clang++
-DCMAKE_C_COMPILER=<path-to-clang-13>/bin/clang'

Release by default builds with -O3 -DNDEBUG. JSC will be built in
WebKitBuild/Release/bin/jsc.

To build the debug version, you must replace --release by --debug, and JSC
will be built in WebKitBuild/Debug/bin/jsc. To rebuild, you can either
remove the WebKitBuild dir, or go in WebKitBuild/Release/ and do a ninja
clean + ninja.

The program I'm using is:

$ cat foo.js
print(Infinity)
let a = Infinity / Infinity
print(Number.isNaN(a))

JSC built in release mode (the value infinity changes every time):

$ WebKitBuild/Release/bin/jsc foo.js
-1.1089394371691584e+269
false

Expected output:
$ ./WebKitBuild/Debug/bin/jsc foo.js
Infinity
true


> I know zero about javascript in general but if we can get a script to
> do that then we could bisect it. It'll take a while but we (Linaro)
> have access to some machines that could help there.
> (assuming this presents on armv8 hardware, but if it doesn't it's at
> least a data point)
>

I'm using:

$ uname -a
Linux bbox-11-armhf 5.10.0-0.bpo.7-arm64 #1 SMP Debian 5.10.40-1~bpo10+1
(2021-06-04) armv8l GNU/Linux


> > It seems like some optimization introduced by -O2 is causing the issue.
>
> Agreed
>
> > Is there a flag in clang to print which optimizations are enabled for
> -O1 and -O2 so I can diff them?
>
> Yes but I can never remember which one it is, so let me try to find
> it. Unless someone else knows it already and can reply.
>
> On Sat, 11 Dec 2021 at 18:01, Mikhail R. Gadelha via cfe-dev
> <cfe-dev at lists.llvm.org> wrote:
> >
> > Hi everyone,
> >
> > This is kind of a follow-up to my previous email about compiling JSC
> using clang for ARMv7: when I build JSC using either -O2 or -O3, I get
> random garbage when querying for the "Infinity" constant from javascript,
> as if the constant was not being initialized. The variable is being
> initialized correctly, that I'm sure.
> >
> > Some tests I did:
> > 1. Using -O1 or no optimization doesn't trigger the issue.
> > 2. Using either -O2 or -O3 with address or the undef behavior sanitizers
> doesn't trigger the issue.
> > 3. Building JSC with clang 11.0.1-2 (from Debian) and clang 12.0.1 (from
> github) doesn't trigger the issue.
> > 4. The issue happens with clang 13.0.0 (from github) and the 13.0.1-rc1
> (also from github).
> >
> > It seems like some optimization introduced by -O2 is causing the issue.
> >
> > Does anyone have any tips I can follow to improve this bug report? I'll
> try to compile JSC with -O2 and disable the optimizations manually to
> pinpoint what's causing the issue (hopefully it's a single optimization and
> not a combination of them). Is there a flag in clang to print which
> optimizations are enabled for -O1 and -O2 so I can diff them?
> >
> > I wish I had more information, but I'm still trying to debug why this is
> happening. I wanted to try to get more information first before opening a
> github issue.
> >
> > Thanks in advance,
> >
> > --
> >
> > Mikhail R. Gadelha.
> > _______________________________________________
> > cfe-dev mailing list
> > cfe-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>


-- 

Mikhail Ramalho.
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