[pkg-golang-devel] Bug#892088: golang-1.10: FTBFS on mips when built on Octeon III buildds

[James Cowgill]

Source: golang-1.10
Version: 1.10-1
Severity: serious
Tags: upstream patch
Forwarded: https://go-review.googlesource.com/c/go/+/97735
X-Debbugs-CC: debian-mips at lists.debian.org

[CC the list for the HW issue]

Hi,

golang-1.10 (and 1.9) FTBFS on mips big-endian with various floating
point test errors, but only when built on the Octeon III buildds.

After tearing my head out investigating this, I have concluded that
there is a hardware bug in the Octeon IIIs. The bug occurs when:
- You run a big floating point double operation (like div.d).
- Wait a few instructions (a store here seems to be important).
- Read the *odd* register which the above float operation stored into.
This ends up reading the value from before the double operation instead
of the result of the operation. I've attached a small C program which
reproduces this. It should happen 99% of the time (it will only fail if
you are unlucky and get an interrupt at the wrong time).

I think this does not affect the wider Debian archive because:
- MFHC1 (move from high float) seems to be unaffected (probably...)
- If mips32r2 and fpxx are enabled, GCC only reads the high part of a
float register using MFHC1 (in the few cases it needs to do this - like
integer <-> float conversion).

However, golang is affected because it uses FP32 and on big endian, all
double loads are split into word loads with the odd register loaded
first. I have attached a patch which fixes this which I have also
submitted upstream.

Thanks,
James
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