[PATCH] D79378: PR34581: Don't remove an 'if (p)' guarding a call to 'operator delete(p)' under -Oz.

[Richard Smith - zygoloid via Phabricator via cfe-commits]

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In D79378#2019336 <https://reviews.llvm.org/D79378#2019336>, @rjmccall wrote:

> Is it reasonable to figure out ahead of time that we can skip the null check completely?  It'd be kindof nice to also take advantage of this at -O0 whenever we don't have real work to do.


In simple cases, we could get a win at `-O0` (deleting an object or array with a trivial (and therefore non-virtual) destructor + non-destroying operator delete + no array cookie). Perhaps in those cases we could sidestep the whole process and just generate a direct call to `operator delete`. That's a different optimization from the one I'm suggesting here (which in turn is trying to be the non-miscompiling subset of the original LLVM-side optimization), though there's lots of overlap. If we don't care about the `-Oz` cases where the destructor is non-trivial before optimization but can be removed by the optimizer, then we could do that instead of this change. If we decide we care about both situations (removing the branch at `-O0` for trivial deletes and removing the branch at `-Oz` for deletes that optimize into being trivial deletes), then I think we need both something like this *and* that check.

What do you think? I'm somewhat inclined to prefer special-casing deletes that the frontend can see are trivial, rather than making the delete unconditional under `-Oz`. That means we might see a code size regression in some cases for `-Oz`, but I suspect that's mostly a theoretical risk.

If we do special-case trivial deletes, should we still insert the branch for them at `-O1` and above? In order for that to be profitable, I think the branch would need to be both well-predicted and usually null, which seems like it could happen but is unlikely to be the common case. I suspect the best answer may be to do this regardless of optimization level.



================
Comment at: clang/lib/CodeGen/CGExprCXX.cpp:2042-2049
   // Null check the pointer.
   llvm::BasicBlock *DeleteNotNull = createBasicBlock("delete.notnull");
   llvm::BasicBlock *DeleteEnd = createBasicBlock("delete.end");
 
   llvm::Value *IsNull = Builder.CreateIsNull(Ptr.getPointer(), "isnull");
 
   Builder.CreateCondBr(IsNull, DeleteEnd, DeleteNotNull);
----------------
dnsampaio wrote:
> Unless I missed something, isn't it better to just avoid emitting this check and basic blocks all together if we are optimizing for size and when we know that Ptr is never null?
> I would consider in doing something alike:
>  ```
>   const bool emitNullCheck = CGM.getCodeGenOpts().OptimizeSize <= 1;
>   llvm::BasicBlock *DeleteNotNull;
>   llvm::BasicBlock *DeleteEnd;
>   if (emitNullCheck){
>     // Null check the pointer.
>     DeleteNotNull = createBasicBlock("delete.notnull");
>     DeleteEnd = createBasicBlock("delete.end");
> 
>     llvm::Value *IsNull = Builder.CreateIsNull(Ptr.getPointer(), "isnull");
> 
>     Builder.CreateCondBr(IsNull, DeleteEnd, DeleteNotNull);
>     EmitBlock(DeleteNotNull);
>   }
> ```
> 
> and we use the same emitNullCheck to avoid EmitBlocks below.
I don't think we can reasonably do this. There are a lot of different ways that `delete` emission can be performed, and many of them (for example, calling a virtual deleting destructor, destroying operator delete, or array delete with cookie) require the null check to be performed in advance for correctness. It would be brittle to duplicate all of those checks here.

We *could* sink the null checks into the various paths through `EmitArrayDelete` and `EmitObjectDelete`, but I think that makes the code significantly more poorly factored.


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