[PATCH] D144334: [Clang] Add C++2b attribute [[assume(expression)]]

[Erich Keane via Phabricator via cfe-commits]

erichkeane added inline comments.


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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.


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  https://reviews.llvm.org/D144334/new/

https://reviews.llvm.org/D144334