[NOT FOR REVIEW] Experimental support for zero-or-trap behavior for uninitialized variables.

[rsmith]

Summary

Add <type> @llvm.trapping.<type>(<type>) intrinsic.

This is intended to produce either the given value or, if we can trace a
use of the value to an instruction with side-effects, a trap.
Notionally, it behaves as a nondeterministic choice between the given
value and poison, where the choice is made angelically such that the
given value is always chosen when the program does not reach a trap.

Simplify trapping(x) and convert it into traps where possible.

Update Clang CodeGen to produce trapping(x) in trivial auto var init
mode.

[rsmith]

• The second operand of 'store' is the pointer, not the first.

[jfb]

Overall I like this approach.

I think it needs three more things to make it:

• Better size optimizations. I measured the code size impact on a codebase which deploys variable auto-init already, and it's a 0.5% code size hit. Considering that auto-init itself was 1%, it means the mitigation now costs 50% more. I haven't dug into why the increase is such, and I assume that there's low-lying fruits.
• I haven't measure performance impact. It might be zero.
• I think we'd need opt remarks support, because we'd want to audit all the places where a trap is left behind.

clang/lib/CodeGen/CGDecl.cpp

1166

Here you need something like:

auto *PointerTy = Ty->getPointerTo(Loc.getAddressSpace());
if (V->getType() != PointerTy)
  Loc = Builder.CreateBitCast(Loc, PointerTy);

Because you can be storing a constant which is layout-compatible with the location, but which doesn't have the same type (say, when we have an explicit padding array of bytes). Line 1184 does that already for other code paths.

[jdoerfert]

FWIW, I tried to solve something similar the other day. My solution sketch looked like this: https://godbolt.org/z/bRQPjd
The idea would be that we teach DSE (and others) to remove the llvm.undef.init intrinsic if the location is overwritten.
In the example above only SROA kicks in properly. I guess this solution is more powerful though.