Bug 394227

Summary:	[x86] False negative "uninitialised value" report due to not recognizing simd registers clear by pcmp*
Product:	[Developer tools] valgrind	Reporter:	myocytebd
Component:	memcheck	Assignee:	Julian Seward <jseward>
Status:	REPORTED ---
Severity:	normal	CC:	ace17, nh2, philippe.waroquiers
Priority:	NOR
Version First Reported In:	3.12.0
Target Milestone:	---
Platform:	Other
OS:	All
Latest Commit:		Version Fixed/Implemented In:
Sentry Crash Report:

Description myocytebd 2018-05-14 04:03:28 UTC

It is common to generate constants in simd registers.
It typically begins by clearing the simd register to all-one, by pcmp* family.
For example:
pcmpeqw xmm0, xmm0

However, valgrind 3.12 (Ubuntu x64) fails to recognize this combination and populates uninitialized values around.
(Changing it to load or load+shuffle will suppress the warning)

Comment 1 Philippe Waroquiers 2018-05-14 20:53:43 UTC

The title of the bug report indicates x86, but below you indicate x64.
Is this a problem with 32 bits intel or 64 bits intel ?

Do you have a small compilable reproducer ?
I am sure that will help the knowledgeable people (i.e. not me :))
to analyse and maybe fix your problem.

Comment 2 ace17 2018-05-23 11:00:11 UTC

Might be related to: https://github.com/libjpeg-turbo/libjpeg-turbo/issues/238

Comment 3 Julian Seward 2018-09-03 06:36:36 UTC

This is probably easy to fix, but we need a test case that shows the
problem for all pcmpeq variants that you are interested in.  Can you
supply one?

Comment 4 nh2 2020-02-03 00:58:56 UTC

The specific case of `pcmpeqw xmm0, xmm0` is claimed to be fixed in issue https://bugs.kde.org/show_bug.cgi?id=290006.

Yet, there are still issues with valgrind on libjpeg-turbo (see https://github.com/libjpeg-turbo/libjpeg-turbo/issues/277#issuecomment-581198382), suggesting that either the fix is incomplete or that other instructions are involved.

Comment 5 nh2 2020-02-03 00:59:29 UTC

Also related: https://bugs.kde.org/show_bug.cgi?id=398153