[cfe-dev] How to configure clang, to get const functions out of the loop (like on FreeBSD) ?

[Nat! via cfe-dev]

I think I know now, what the problem is. This summary is also in 
https://bugs.llvm.org/show_bug.cgi?id=34208#c4 preceeded by a lot of 
other comments, how I got there.

Note: The loop header is essentially the loop body in this test case.

Summary
=======

A plain C function `bar` answers NO to I.mayThrow(), but YES to 
isGuaranteedToTransferExecutionToSuccessor(&*I). This poisons the whole 
loop header as SafetyInfo->HeaderMayThrow gets set to YES.

(See: llvm::computeLoopSafetyInfo 
https://github.com/llvm-mirror/llvm/blob/987e30d8673d9d1a683ef811e9b7e32d2fadeb8a/lib/Transforms/Scalar/LICM.cpp#L496)

When llvm::canSinkOrHoistInst asks llvm::isGuaranteedToExecute about foo 
(via isSafeToExecuteUnconditionally), this poisioning of the loop header 
inhibits the hoisting of this const function (with constant argument). 
(https://github.com/llvm-mirror/llvm/blob/987e30d8673d9d1a683ef811e9b7e32d2fadeb8a/lib/Transforms/Scalar/LICM.cpp#L483)


```
   if (Inst.getParent() == CurLoop->getHeader())
     // If there's a throw in the header block, we can't guarantee we'll 
reach
     // Inst.
     return !SafetyInfo->HeaderMayThrow;
```

The setting SafetyInfo->HeaderMayThrow is now overly broad. Ideally I 
think loop headers should be split into a non-throw part and a throw part.

As an alternative non-throwing const Functions with constant arguments, 
could always be considered hoistable regardless of 
SafetyInfo->HeaderMayThrow.
---


So what to do next ?

Ciao
    Nat!