linux/loong64: Go 1.24.0 issue report

[Jan Mercl]

Hi there,

It seems go1.24.0 introduced a regression on linux/loong64.

ccgo failure log: https://gitlab.com/cznic/builder/-/raw/master/logs/modernc.org/ccgo/v4/lib/linux_loong64b

jnml@e5-1650:~/src/modernc.org/builder$ ./bisect.sh ccgo loong
modernc.org/ccgo/v4/lib|linux_loong64b|2025-03-07 08:42:40+08:00|ef89cd4a4d446681edd8309530326bcade1fe0b7|linux|loong64|PASS|go1.23.5|
modernc.org/ccgo/v4/lib|linux_loong64b|2025-03-08 13:21:55+08:00|36bb23fb51ff4dd1a36dcff652fabc244787cdc5|linux|loong64|FAIL|go1.24.1|
jnml@e5-1650:~/src/modernc.org/builder$

Reproducer: https://go.dev/play/p/GGKWSiC-IDs

Steps to reproduce:

[cznic@cznic-loong64-01 ccgo]$ ~/bin/go1.23.5/bin/go run main.go
11 -0.08333333333333337
12 0
13 0.08333333333333326
[cznic@cznic-loong64-01 ccgo]$ ~/bin/go1.24.0/bin/go run main.go
11 -0.08333333333333338
12 -5.551115123125783e-17
13 0.08333333333333327
[cznic@cznic-loong64-01 ccgo]$ ~/bin/go1.24.1/bin/go run main.go
11 -0.08333333333333338
12 -5.551115123125783e-17
13 0.08333333333333327
[cznic@cznic-loong64-01 ccgo]$ ~/bin/go1.24.2/bin/go run main.go
11 -0.08333333333333338
12 -5.551115123125783e-17
13 0.08333333333333327
[cznic@cznic-loong64-01 ccgo]$ uname -a
Linux cznic-loong64-01 6.12.0-0.rc5.20241031git0fc810ae3ae1.47.fc38.loongarch64 #1 SMP PREEMPT_DYNAMIC Fri Nov  1 15:47:31 CST 2024 loongarch64 GNU/Linux
[cznic@cznic-loong64-01 ccgo]$

Someone please take a look, thanks.

-j

[Keith Randall]

Probably just FMA from https://go-review.googlesource.com/c/go/+/625335

Try turning FMA off to see if it reproduces the old behavior. `-gcflags=-d=fmahash=n`, maybe?

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[Jan Mercl]

On Wed, Apr 9, 2025 at 5:44 PM Keith Randall <keith....@gmail.com> wrote:

> Probably just FMA from https://go-review.googlesource.com/c/go/+/625335
> Try turning FMA off to see if it reproduces the old behavior. `-gcflags=-d=fmahash=n`, maybe?

You're right, thanks!

Wrapping x*y into floatNN(x*y) is easy to add to the C transpiler and
I manually verified it works in this case. I hope it will not have
other cascading side effects.

Thanks again,

-j

[sophie zhao]

Thank you for the feedback. We have reproduced the issue and are currently analyzing it to find a solution.