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Alyssa Rosenzweig 298b19e70f nir: fix miscompiles with rules with INT32_MIN 
812b3415 added rules for upcasts with comparisons with a variety of
types. The float & unsigned rules should be ok, but the signed integer rules are
unsound as currently implemented. This can cause end-to-end miscompiles.

I originally hit this issue while debugging a large real world OpenCL kernel. I
found the bug symptoms changed when disabling loop unrolling, which tipped me
off to a compiler bug. I've reduced it to a minimal test case. Imagine my
surprise when I find out the NIR my backend ingested was already constant folded
to be wrong.

In the minimal test case, during optimization we have NIR:

        32     %6 = ....
        64     %9 = i2i64 %6
        64    %44 = load_const (0x0000000000000001)
        1     %45 = ilt %9, %44 (0x1)

This is a simple check (int64_t)%6 < 1.

nir_opt_algebraic turns this into:

        32     %6 = ...
        64     %9 = i2i64 %6
        64    %44 = load_const (0x0000000000000001)
        64    %55 = load_const (0x0000000080000000 = 2147483648)
        1     %56 = ilt %55 (0x80000000), %44 (0x1)
        64    %57 = load_const (0x000000007fffffff = 2147483647)
        1     %58 = ilt %57 (0x7fffffff), %44 (0x1)
        32    %59 = i2i32 %44 (0x1)
        1     %60 = ilt %6, %59
        1     %61 = ior %58, %60
        1     %62 = iand %56, %61

This pile of math constant-folds to an unconditional "false"!  The problem is
%56. At first glance, INT32_MIN < 1 is true so %56 should be true. Indeed, it
should. But here's the kicker: both constants are 64-bit here, so the ilt
operation is a 64-bit comparison -- that left-hand side is INT32_MIN
zero-extended to 64-bit for the signed comparison at 64-bit. So in fact, it
evaluates to false, causing the whole expression to go false.  If we're going to
do a 64-bit comparison for %56, then we need to sign-extend the bound.  So we'll
just adjust the Python and be on our way, right?

Unfortunately the issue is deeper. According to the comment in the generated
nir_opt_algebraic.c file, the guilty algebraic rule is:

   ('ilt', ('i2i64', 'a@32'), '#b') =>
   ('iand', ('ilt', -2147483648, 'b'), ('ior', ('ilt', 2147483647, 'b'), ('ilt', 'a', ('i2i32', 'b'))))

From a Python perspective? That rule is correct. -2147483648 < 1 is a true
statement. Adjusting the Python rule is not the appropriate solution here, since
the issue is more fundamental and might affect other rules.  The real problem is
the translation of that Python replacement tree into C, incorrectly
zero-extending -2147483648 into 0x0000000080000000 instead of sign-extending to
0xffffffff80000000.

Crawling down the rabbit hole of the generated algebraic file, we see the
constant encoded as:

   { .constant = {
      { nir_search_value_constant, 64 },
      nir_type_int, { -0x80000000 /* -2147483648 */ },
   } },

NIR correctly translates the negative constant to a C level negate operation of
its absolute value. This maps to the correct sign-extension...

...for all constants except for INT_MIN. Because that constant lacks a ULL
suffix, it is a 32-bit integer. And for this integer (only), negating it hits
signed integer overflow (UB!) and then we end up with an effective
zero-extension when going to 64-bit.

This patch fixes the end-to-end miscompile.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Closes: #11402
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29952>
(cherry picked from commit 270446ee21)
2024-07-02 18:01:49 +02:00
.ci-farms ci: re-enable Igalia farm 2024-04-17 01:23:25 +00:00
.ci-farms-disabled ci: re-enable Igalia farm 2024-04-17 01:23:25 +00:00
.github/workflows add zink to macos ci 2022-11-22 19:04:13 +00:00
.gitlab gitlab: make commit more commit-like formatted 2023-11-09 17:30:07 +00:00
.gitlab-ci ci: fix section_end in debian-build-testing 2024-06-19 20:05:45 +02:00
android nouveau: import libdrm_nouveau 2024-03-13 15:21:07 +00:00
bin ci_run_n_monitor: explain how to pass multiple targets without having to use regexes 2024-03-29 19:23:24 +00:00
build-support meson: move tsan-blacklist.txt to build-support with the other build support files 2024-05-01 13:21:56 +02:00
docs docs: fix bootstrap-extension 2024-06-26 22:08:44 +02:00
include dri: rename 'implicit' param from earlier series 2024-06-21 17:50:04 +02:00
src nir: fix miscompiles with rules with INT32_MIN 2024-07-02 18:01:49 +02:00
subprojects subprojects: uprev perfetto to v45.0 2024-05-22 10:35:33 +02:00
.clang-format meson: enable the clang-format target 2023-05-29 11:57:08 +00:00
.clang-format-ignore ci: enforce formatting for RADV & ACO 2023-06-16 19:59:52 +00:00
.clang-format-include teflon: Initial commit 2024-01-24 10:02:10 +00:00
.dir-locals.el dir-locals.el: Adds White Space support 2016-11-14 19:17:49 +02:00
.editorconfig glsl: Fixes ident issue in glsl_parser.yy and update editorconfig for it 2022-12-16 19:02:17 +00:00
.git-blame-ignore-revs freedreno: Add reformatting commits to .git-blame-ignore-revs 2023-09-22 02:07:36 +00:00
.gitattributes gitlab: Highlight .cl as C 2023-11-02 11:37:46 +00:00
.gitignore .gitignore: Add .venv folder 2024-01-24 10:53:14 +00:00
.gitlab-ci.yml ci: identify and label S3 buckets 2024-05-15 15:37:05 +02:00
.graphqlrc.yml ci/bin: Add utility to find jobs dependencies 2022-08-03 23:10:37 +00:00
.mailmap .mailmap: add an alias for Christian Gmeiner 2023-06-08 09:16:25 +00:00
.mr-label-maker.yml mr-label-maker: be explicit about the various CI files 2024-04-08 13:33:51 +00:00
.pick_status.json nir: fix miscompiles with rules with INT32_MIN 2024-07-02 18:01:49 +02:00
CODEOWNERS CODEOWNERS: update Imagination maintainers 2024-01-19 10:26:15 +00:00
meson.build clover: fix meson opencl-spirv option 2024-07-01 01:37:25 +02:00
meson_options.txt meson: implement split-debug 2024-04-23 18:31:39 +00:00
README.rst docs: promote #dri-devel on oftc over freenode 2021-05-24 09:21:48 +00:00
VERSION VERSION: bump for 24.1.2 2024-06-19 18:12:20 +02:00

README.rst 

`Mesa <https://mesa3d.org>`_ - The 3D Graphics Library
======================================================


Source
------

This repository lives at https://gitlab.freedesktop.org/mesa/mesa.
Other repositories are likely forks, and code found there is not supported.


Build & install
---------------

You can find more information in our documentation (`docs/install.rst
<https://mesa3d.org/install.html>`_), but the recommended way is to use
Meson (`docs/meson.rst <https://mesa3d.org/meson.html>`_):

.. code-block:: sh

  $ mkdir build
  $ cd build
  $ meson ..
  $ sudo ninja install


Support
-------

Many Mesa devs hang on IRC; if you're not sure which channel is
appropriate, you should ask your question on `OFTC's #dri-devel
<irc://irc.oftc.net/dri-devel>`_, someone will redirect you if
necessary.
Remember that not everyone is in the same timezone as you, so it might
take a while before someone qualified sees your question.
To figure out who you're talking to, or which nick to ping for your
question, check out `Who's Who on IRC
<https://dri.freedesktop.org/wiki/WhosWho/>`_.

The next best option is to ask your question in an email to the
mailing lists: `mesa-dev\@lists.freedesktop.org
<https://lists.freedesktop.org/mailman/listinfo/mesa-dev>`_


Bug reports
-----------

If you think something isn't working properly, please file a bug report
(`docs/bugs.rst <https://mesa3d.org/bugs.html>`_).


Contributing
------------

Contributions are welcome, and step-by-step instructions can be found in our
documentation (`docs/submittingpatches.rst
<https://mesa3d.org/submittingpatches.html>`_).

Note that Mesa uses gitlab for patches submission, review and discussions.