erichkeane added inline comments.
================
Comment at: clang/lib/CodeGen/CGStmt.cpp:723-727
+ case attr::Assume: {
+ llvm::Value *ArgValue = EmitScalarExpr(cast<AssumeAttr>(A)->getCond());
+ llvm::Function *FnAssume = CGM.getIntrinsic(llvm::Intrinsic::assume);
+ Builder.CreateCall(FnAssume, ArgValue);
+ break;
----------------
Izaron wrote:
> cor3ntin wrote:
> > I'm not familiar with codegen. what makes the expression non-evaluated here?
> The LLVM docs says it:
> https://llvm.org/docs/LangRef.html#llvm-assume-intrinsic
> > No code is generated for this intrinsic, and instructions that contribute
> > only to the provided condition are not used for code generation
> The `ArgValue` and `FnAssume` are the instructions to be discarded according
> to this sentence.
>
> I guess they do it via dead code elimination (at any optimization level).
So interestingly, with __builtin_assume, we have some code that checks for
side-effects (https://godbolt.org/z/fo73TvY68) and just doesn't emit the assume
at all into IR. I believe we are not able to do that (as side-effects don't
prevent attribute-assume from having 'an effect'), so additional work needs to
happen to make sure an example like the above has those side effects removed,
but considered by the backend.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D144334/new/
https://reviews.llvm.org/D144334
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