[LLVMdev] "Best" alias analysis algorithm
ghost at cs.msu.su
Mon Apr 25 05:09:42 PDT 2005
On Monday 25 April 2005 14:43, Vladimir Prus wrote:
> The 'i' variable is never modified in the program, however, all analyses
> except for -globalsmodref-aa report that the
> %tmp.3 = call int %_Z3bari( int %p ) ; <int> [#uses=1]
> instruction can modify 'i'. I'm somewhat surprised, because it looks like
> -globalsmodref-aa is the simplest algorithm. So, why the order algorithms
> report possible modification?
> In fact, if I add
> %j = external global int ;
> which is not used *anywhere*, most algorithms report that %j is modified by
> the call. Am I missing something, or -globalsmodref-aa is the only alias
> analysis that properly handles globals?
Things are actually worse. If I declare two global variables, and modify the
called function so that it changes just one variable, then alias analysis
(with --globalsmodref-aa) reports that the call instruction can write to both
The C++ program is at
The LLVM bytecode is at
And the output from my code is
The code itself is in
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)))
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.
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