[llvm-commits] [llvm] r158937 - in /llvm/trunk: include/llvm/Analysis/MemoryBuiltins.h lib/Analysis/MemoryBuiltins.cpp lib/Transforms/InstCombine/InstCombineCalls.cpp lib/Transforms/InstCombine/InstructionCombining.cpp test/Transforms/InstCombine

[Nuno Lopes]

Quoting Nick Lewycky <nlewycky at google.com>:

> On 21 June 2012 14:25, Nuno Lopes <nunoplopes at sapo.pt> wrote:
>
>> Author: nlopes
>> Date: Thu Jun 21 16:25:05 2012
>> New Revision: 158937
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=158937&view=rev
>> Log:
>> Add support for invoke to the MemoryBuiltin analysid.
>> Update comments accordingly.
>>
>> Make instcombine remove useless invokes to C++'s 'new' allocation function
>> (test attached).
>>
>
> Note that instcombine has "setPreservesCFG" set, which means that it really
> shouldn't be touching any terminator instructions. It currently does to
> swap things like "br i1 (xor b, true), label %l1, label %l2" and arguably
> that doesn't really change the CFG (though I think that in an ideal world
> instcombine should not be doing that either). This patch certainly does
> though, and instcombine shouldn't be doing it. You could put it in
> SimplifyCFG maybe?

Right, just noticed that setPreservesCFG bit as well..
First I was thinking to create an artificial edge with 'br i1 true,  
normalDest, unwindDest', but LLVM's verifier rejects that.
This trick is actually used in another place in instcombine, which  
means that the following code crashes instcombine:
   %call = invoke noalias i8* undef()
           to label %invoke.cont unwind label %lpad

So now we have 2 places in instcombine that are trying to remove  
invoke instructions..
A simple solution could be to replace 'invoke @malloc' with 'invoke  
undef', and then move the logic to remove these invokes to  
SimplifyCFG.  What do you think?

Nuno