[PATCH] Simplify and correct landing pad lowering

[Jakob Stoklund Olesen]

On Jul 3, 2013, at 4:25 PM, Anton Korobeynikov <anton at korobeynikov.info> wrote:

> Hi Jakob,
> 
> Looks extremely good solution to me!

Thanks, Anton.

Committed as r185594-r185596.

/jakob

> On Thu, Jul 4, 2013 at 3:17 AM, Jakob Stoklund Olesen <stoklund at 2pi.dk> wrote:
>> Hi,
>> 
>> Please review these patches which fix a problem with our landing pad lowering and remove some unnecessary abstraction.
>> 
>> DWARF-style landing pads receive two arguments in registers: The exception pointer (%rax) and the exception selector (%rdx). Currently, these two arguments are represented by the abstract ISD::EXCEPTIONADDR and ISD::EHSELECTOR SDNodes. All in-tree targets legalize these nodes by expanding them to CopyFromReg nodes reading the corresponding physregs (%rax and %rdx).
>> 
>> This doesn’t actually work correctly because there is no dependency preventing the %rax and %rdx registers from being clobbered before the CopyFromReg nodes are scheduled. A landing pad with a DIV instruction could easily do that:
>> 
>> BB#4: derived from LLVM BB %lpad.i.i, EH LANDING PAD
>>    Live Ins: %EAX %EDX
>>    Predecessors according to CFG: BB#3
>> ..
>>        DIV %EAX<imp-def>, %EDX<imp-def>; Live-in values clobbered.
>> ..
>>        %vreg28<def> = COPY %EAX<kill>; GR32:%vreg28
>>        %vreg29<def> = COPY %EDX<kill>; GR32:%vreg29
>> 
>> 
>> These patches change the handling of landing pad arguments to look more like the handling of normal function arguments. The live-in physical registers are immediately copied into virtual registers at the top of the basic block. The CopyFromReg nodes are changed to read the virtual registers instead of reading the live-in physregs directly.
>> 
>> I also went ahead and removed the EXCEPTIONADDR, EHSELECTOR, and LSDAADDR ISD opcodes. They don’t seem to provide a useful abstraction in addition to the TLI.setExceptionPointerRegister() / TLI.setExceptionSelectorRegister() functions, and all targets use the default ‘Expand’ legalization anyway.
>> 
>> Thanks,
>> /jakob
>> 
>> 
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>> 
> 
> 
> 
> -- 
> With best regards, Anton Korobeynikov
> Faculty of Mathematics and Mechanics, Saint Petersburg State University