[LLVMdev] "Best" alias analysis algorithm

Chris Lattner sabre at nondot.org
Mon Apr 25 12:40:54 PDT 2005

On Mon, 25 Apr 2005, Vladimir Prus wrote:
> The GlobalsModRef::getModRefInfo has this logic:
>    // If we are asking for mod/ref info of a direct call with a pointer to a
>    // global we are tracking, return information if we have it.
>    if (GlobalValue *GV = const_cast<GlobalValue*>(getUnderlyingObject(P)))
>       if (GV->hasInternalLinkage())
> So, no information is produced for external variables, the function calls
> Aliasanalysis::getModRefInfo, which sees that called function may write to
> memory, and returns true for all global variables.
> Anything I can do about it? What I what is minimally accurate information
> about all global variables a function may modify. "Modifies them all" is
> clearly not even minimally accurate.

Oh ok, now I remember.  The deal here is that "globals-modref" is supposed 
to be a very simple mod/ref analysis that does no "pointer analysis" at 
all.  In particular, it only tracks globals that it knows "never have 
their address taken".  If a global has non-internal linkage, the compiler 
can't know that something outside of the program doesn't take its address 
and pass around its address.

While in this case, globals modref could handle this case, you really want 
something more powerful like -ds-aa.



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