[PATCH] D60942: Emit diagnostic if inline asm "n" constraint isn't an immediate

Mon May 13 18:11:31 PDT 2019

joerg added a comment.

In D60942#1500742 <https://reviews.llvm.org/D60942#1500742>, @void wrote:

> In D60942#1500730 <https://reviews.llvm.org/D60942#1500730>, @joerg wrote:
>
> > On the frontend side:
> >
> > - __builtin_constant_p is not expanded correctly in EvalConstant.cpp when used as part of codegen for -O0. This means that trivially unreachable branches are emitted (e.g. with a BR with a literal false as condition),
> > - The constant evaluator has a general problem with overly pessimistic analysis when it comes to possible side effects. Still have to discuss the correct handling with Richard for this.
>
>
> GCC has this to say about `__builtin_constant_p`'s behavior:
>
> > A return of 0 does not indicate that the value is not a constant, but merely that GCC cannot prove it is a constant with the specified value of the -O option.
>
> When I was modifying our implementation recently, I made it so that if there are no optimizations then we evaluate to `0` in the front-end before code-gen. It might be that those blocks are being emitted, but they should have something like `br i1 false, label %true_bb, label %false_bb` or equivalent.

Yes, that's correct. The problem is that with `-O0`, we never run SimplifyCFG, so those conditional branches make it straight to the backend. As discussed with Richard on IRC, the core issue is that for -O0, the IR generation for __builtin_constant_p is different from the constant folding as part of AST/EvalConstant.cpp. The latter is used by EmitIfStmt and friends to decide whether the BBs should be created in first place and currently fails evaluation for bcp.

>> On the backend side:
>> 
>> - The is_constant intrinsic is lowered too late. The prepare-isel pass it is currently part of is run after the last SimplifyCFG pass, so the now-dead basic blocks are never pruned and therefore the asm statements survive.
> 
> Is this at `-O0` or all optimization levels?

With `-O0`, the intrinsic is never generated and no SimplifyCFG is run either (see above).

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https://reviews.llvm.org/D60942