[LLVMdev] malloc vs malloc

[Chris Lattner]

On Jan 12, 2009, at 7:45 PM, Nick Lewycky wrote:
>> isa<FreeInst>(X) can be replaced with:
>>
>> bool isFree(Instruction *X) {
>>   if (CallInst *CI = dyn_cast<CallInst>(X))
>>     if (Function *F = CI->getCalledFunction())
>>       if (F->isName("free") && F->hasExternalLinkage())
>
> Surely you mean "llvm.free" or something, right? I don't think we want
> to start assigning meaning to otherwise arbitrary function names.

No, I mean "free".  Dan Gohman and I discussed this today.  A short  
version in no particular order is:

1) in practice, IMO, everyone building for user space has libc in  
their address space, even if they are supporting some non-C language  
like haskell or ocaml or something.
2) in practice, IMO, if an app is in kernel space or embedded, if they  
use a symbol (e.g. printf) it almost certainly does what ansi says it  
does.
3) I really think the functionality of simplifylibcalls should be  
moved into instcombine someday, so that it is run more than once and  
is iterative.
4) we want other passes to be able to optimize libcalls.  For example,  
the stuff I added in r61918 requires GVN or memcpyopt to optimize  
things like strlen.
5) regardless of #1/2, we want to support things like -fno-builtin- 
free someday.  -ffree-standing would also imply this sort of thing, as  
would -fno-builtins etc.
6) there are other routines that we want to make assumptions about,  
that are almost certainly true, but are not standard conformant.  I'm  
thinking things like "operator new is sane and thus the result is  
noalias".  This specific example should be controlled by something  
like -foperator-new-is-sane, and probably default to on.

To me, there are two solutions.  One bad solution, for various  
reasons, is to create an llvm.foo for every foo function we want to  
optimize.  IMO, a better solution is to introduce *one* new function  
attribute "is normal" or "may not be normal" (name suggestions  
welcome, pick your parity) which can be set on a function.  All  
existing libcall optimization stuff would be predicated on the  
function being named the right thing, external, and "normal".  This  
would allow language independent optimizations like instcombine to do  
this stuff.

-Chris