[PATCH] D102817: [Windows SEH]: HARDWARE EXCEPTION HANDLING (MSVC -EHa) - Part 2

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.


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Comment at: llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp:1316
+        //  Div with variable opnds won't be the first instruction in
+        //  an EH region as it must be led by at least a Load
+        {
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tentzen wrote:
> efriedma wrote:
> > If the issue here that the "end" label for a region can accidentally include the following instruction after the region?  Would it make sense to explicitly insert a nop earlier, in that case?
> I don't see the value of emitting it earlier. I think the closer to emission is safer. Or there is always a risk of being changed or reordered by other phases. Besides, there are a couple of similar cases of emitting Nop in that file (like functionPrfix). I think inserting the nop where the EH_Label is emitted is safest and most straight-forward.
Could you at least clarify the comment to clarify what aspect of the SEH region is problematic?  We currently use EH_LABEL for both the beginning and end of a region, and it isn't clear which one this is meant to apply to.


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Comment at: llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp:2916
+      if (CleanupMBB) // a block referenced by EH table
+        CleanupMBB->setHasAddressTaken(); // so dtor-funclet not removed by opts
       break;
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tentzen wrote:
> efriedma wrote:
> > I assume this is trying to trick optimizations into avoiding optimizations on the cleanup block if the __try involves multiple basic blocks.
> > 
> > Have you considered trying to fix the control flow graph?  I mean, I understand the difficulty of expressing the control flow at the IR level, given we don't have IR versions of "load" and "store" with an unwind edge, but those concerns don't really apply in the same way in a MachineFunction.  You can mark the cleanup block as a successor of any block that can throw an exception, without explicitly marking up the control flow on each individual instruction.  (This is how exceptions normally work.)
> The Cleanup block is turned into a dtor funclet at the end. And the beginning address of that funclet will be recorded in EH table in .xdata section. So it's really address-taken by definition. If you verify this by checking Linker relocation records or the EH table in .asm file.  IMO this flag should be set regardless of the SEH feature.
That's not what "address-taken" is supposed to mean.

It's supposed to be equivalent to BasicBlock::hasAddressTaken, which means that the block is the target of a blockaddress expression (used for indirectbr).  Primarily, this restricts certain optimizations because in general, we can't split a critical edge from an indirectbr.

hasAddressTaken() is not a general "will the address of the block eventually be emitted somewhere in the assembly".  That can happen for a variety of unrelated reasons, including exception handling, debug info, jump tables, and branch lowering.


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  https://reviews.llvm.org/D102817/new/

https://reviews.llvm.org/D102817