commit b98a0c7f6c9c7b38b6cd764e6f47e16e3d8c342c
author Roman Lebedev <lebedev.ri@gmail.com> Wed Nov 27 17:07:06 2019 +0300
committer Roman Lebedev <lebedev.ri@gmail.com> Wed Nov 27 21:52:41 2019 +0300
tree 5630f911af69764e8260d7e9d4d5a514586fd8c1
parent ebfff46c8d29efd9767a24043766ddd588db26c3

[clang][CodeGen] Implicit Conversion Sanitizer: handle increment/decrement (PR44054)(take 2)

Summary:
Implicit Conversion Sanitizer is *almost* feature complete.
There aren't *that* much unsanitized things left,
two major ones are increment/decrement (this patch) and bit fields.

As it was discussed in
[[ https://bugs.llvm.org/show_bug.cgi?id=39519 | PR39519 ]],
unlike `CompoundAssignOperator` (which is promoted internally),
or `BinaryOperator` (for which we always have promotion/demotion in AST)
or parts of `UnaryOperator` (we have promotion/demotion but only for
certain operations), for inc/dec, clang omits promotion/demotion
altogether, under as-if rule.

This is technically correct: https://rise4fun.com/Alive/zPgD
As it can be seen in `InstCombineCasts.cpp` `canEvaluateTruncated()`,
`add`/`sub`/`mul`/`and`/`or`/`xor` operators can all arbitrarily
be extended or truncated:
https://github.com/llvm/llvm-project/blob/901cd3b3f62d0c700e5d2c3f97eff97d634bec5e/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp#L1320-L1334

But that has serious implications:
1. Since we no longer model implicit casts, do we pessimise
   their AST representation and everything that uses it?
2. There is no demotion, so lossy demotion sanitizer does not trigger :]

Now, i'm not going to argue about the first problem here,
but the second one **needs** to be addressed. As it was stated
in the report, this is done intentionally, so changing
this in all modes would be considered a penalization/regression.
Which means, the sanitization-less codegen must not be altered.

It was also suggested to not change the sanitized codegen
to the one with demotion, but i quite strongly believe
that will not be the wise choice here:
1. One will need to re-engineer the check that the inc/dec was lossy
   in terms of `@llvm.{u,s}{add,sub}.with.overflow` builtins
2. We will still need to compute the result we would lossily demote.
   (i.e. the result of wide `add`ition/`sub`traction)
3. I suspect it would need to be done right here, in sanitization.
   Which kinda defeats the point of
   using `@llvm.{u,s}{add,sub}.with.overflow` builtins:
   we'd have two `add`s with basically the same arguments,
   one of which is used for check+error-less codepath and other one
   for the error reporting. That seems worse than a single wide op+check.
4. OR, we would need to do that in the compiler-rt handler.
   Which means we'll need a whole new handler.
   But then what about the `CompoundAssignOperator`,
   it would also be applicable for it.
   So this also doesn't really seem like the right path to me.
5. At least X86 (but likely others) pessimizes all sub-`i32` operations
   (due to partial register stalls), so even if we avoid promotion+demotion,
   the computations will //likely// be performed in `i32` anyways.

So i'm not really seeing much benefit of
not doing the straight-forward thing.

While looking into this, i have noticed a few more LLVM middle-end
missed canonicalizations, and filed
[[ https://bugs.llvm.org/show_bug.cgi?id=44100 | PR44100 ]],
[[ https://bugs.llvm.org/show_bug.cgi?id=44102 | PR44102 ]].

Those are not specific to inc/dec, we also have them for
`CompoundAssignOperator`, and it can happen for normal arithmetics, too.
But if we take some other path in the patch, it will not be applicable
here, and we will have most likely played ourselves.

TLDR: front-end should emit canonical, easy-to-optimize yet
un-optimized code. It is middle-end's job to make it optimal.

I'm really hoping reviewers agree with my personal assessment
of the path this patch should take..

This originally landed in 9872ea4ed1de4c49300430e4f1f4dfc110a79ab9
but got immediately reverted in cbfa237892e55b7129a1178c9b03f26683d643af
because the assertion was faulty. That fault ended up being caused
by the enum - while there will be promotion, both types are unsigned,
with same width. So we still don't need to sanitize non-signed cases.
So far. Maybe the assert will tell us this isn't so.

Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=44054 | PR44054 ]].
Refs. https://github.com/google/sanitizers/issues/940

Reviewers: rjmccall, erichkeane, rsmith, vsk

Reviewed By: erichkeane

Subscribers: mehdi_amini, dexonsmith, cfe-commits, #sanitizers, llvm-commits, aaron.ballman, t.p.northover, efriedma, regehr

Tags: #llvm, #clang, #sanitizers

Differential Revision: https://reviews.llvm.org/D70539