NB: by "unsigned()" above I meant either uint() , uvec2(), uvec4(), uvec4().

CC'ing Olli, who has a lot of experience with shader compiler issues.

GLSL ES 3.00.6 section 5.4.1: "It is undefined to convert a negative floating point value to an uint". It's the same also in more recent versions.

That said it would be helpful to ensure that ANGLE is not using any undefined C++ behavior in its constant folding. Also we can issue a warning in case a negative float is being cast to unsigned int. Here's a patch:

https://chromium-review.googlesource.com/c/angle/angle/+/1026681

anyway there is a full load of operations where result differs totally between const float and unconst (and not just the precision).
This is ultra-dangerous since during and at the end of a project it's not rare to switch some parameters between const or not, and the effect might not show up immediately. 
I know the GLSL spec is just a big bag of holes, but maybe at least some constructors and Angle team might be interested in one upper-level of reliability ?  
( very seniored might all pile-up uint(int(floor(x))) to be safe, but how many unprotected lines of code around ?)

The following revision refers to this bug:
  https://chromium.googlesource.com/angle/angle/+/f827123d99457a294c3d9451fe3af69772f0bcc2

commit f827123d99457a294c3d9451fe3af69772f0bcc2
Author: Olli Etuaho <oetuaho@nvidia.com>
Date: Wed Apr 25 14:21:03 2018

Handle negative float to uint conversion robustly

Converting a negative float to uint is undefined in the GLSL ES 3.00.6
spec.

However, it improves portability if we don't trigger undefined results
in C++ in this case. To do this, we cast negative floats first to
signed integer before casting them to unsigned integer.

We also issue a warning about an undefined conversion in case a
negative float was converted to uint.

BUG= chromium:835868 
TEST=angle_unittests

Change-Id: I9835a739ec80699d420a4f91a3bfa112c9a13604
Reviewed-on: https://chromium-review.googlesource.com/1026681
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>

[modify] https://crrev.com/f827123d99457a294c3d9451fe3af69772f0bcc2/src/tests/compiler_tests/ConstantFolding_test.cpp
[modify] https://crrev.com/f827123d99457a294c3d9451fe3af69772f0bcc2/src/compiler/translator/IntermNode.cpp
[modify] https://crrev.com/f827123d99457a294c3d9451fe3af69772f0bcc2/src/compiler/translator/ConstantUnion.cpp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d93a64651183f6f6117b442ea2aea74aab3e32cc

commit d93a64651183f6f6117b442ea2aea74aab3e32cc
Author: angle-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com <angle-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Date: Wed Apr 25 22:41:26 2018

Roll src/third_party/angle/ 10434f676..23dc90b81 (5 commits)

https://chromium.googlesource.com/angle/angle.git/+log/10434f67672d..23dc90b81efb

$ git log 10434f676..23dc90b81 --date=short --no-merges --format='%ad %ae %s'
2018-04-23 lfy GLES1: Enable/disable for texture targets
2018-04-25 oetuaho Handle negative float to uint conversion robustly
2018-04-24 lucferron Vulkan: Enable UnpackAlignmentTest and remove useless warning.
2018-04-24 lucferron Vulkan: Fix issue with texsubimage2d barriers
2018-04-25 jmadill Clean up DEPS style.

Created with:
  roll-dep src/third_party/angle
BUG= chromium:835868 , chromium:782846 


The AutoRoll server is located here: https://angle-chromium-roll.skia.org

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.


CQ_INCLUDE_TRYBOTS=luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
TBR=geofflang@chromium.org

Change-Id: I7ff0b0ad560ed9fadcce00db4c136232330826c9
Reviewed-on: https://chromium-review.googlesource.com/1028521
Commit-Queue: angle-chromium-autoroll <angle-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Reviewed-by: angle-chromium-autoroll <angle-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#553799}
[modify] https://crrev.com/d93a64651183f6f6117b442ea2aea74aab3e32cc/DEPS

There's not much more we can feasibly do to make this more consistent. Closing as WontFix since this case is not defined in the spec.

Adding blocked on bug angleproject:3064, an alternative to defining the undefined behavior