Hi Everyone,
I asked a question on the dev list related to the topic to which John Criswell and Jeremy Lakeman kindly provided some valuable insight.
I’m still stuck on the issue and i’m hoping i didn’t phrase the question well enough.
Usual practice is to continue the discussion in the original thread if the question is not a new one.
I have a foo.c file that is :
#include <stdio.h>
int foo(int a, int b){
return a+b;
}
int main() {
int x=foo(3,1);
printf("%d\n",x);
return 0;
}
Now, i obtain the foo.bc/foo.ll file,
the code i’m interested in looks like:
; Function Attrs: norecurse nounwind readnone uwtable
define i32 @addd(i32 %a, i32 %b) #0 {
entry:
%add = add nsw i32 %b, %a
ret i32 %add
}
running the file with lli foo.ll outputs
4
Now we know that the values of %a and %b are 3 and 1 respectively
I’m not sure what you mean with this sentence. lli just interpreted the file, starting in main() and executing instruction one after each other.
, is there any way i can retrieve these values by running foo.bc through a pass??
I know i->getOperand(0) would get me i32 %a
i->getOperand(1) would get me i32 %b
How do i retrieve 3 and 1, the integer values that these operands hold ?
3 and 1 are arguments at the call site, while %a and %b are the actual parameter to the function foo(). You need to inspect the call site (there are potentially/usually multiple call-sites) if you have a pointer to the function foo().
The call sites will be amongst the user of the Function. The best is to look for example in the LLVM codebase, for instance:
/// AllCallersPassInValidPointerForArgument - Return true if we can prove that
/// all callees pass in a valid pointer for the specified function argument.
static bool AllCallersPassInValidPointerForArgument(Argument *Arg) {
Function *Callee = Arg->getParent();
const DataLayout &DL = Callee->getParent()->getDataLayout();
unsigned ArgNo = Arg->getArgNo();
// Look at all call sites of the function. At this pointer we know we only
// have direct callees.
for (User *U : Callee->users()) {
CallSite CS(U);
assert(CS && “Should only have direct calls!”);
if (!isDereferenceablePointer(CS.getArgument(ArgNo), DL))
return false;
}
return true;
}
Visible here as well: http://llvm.org/docs/doxygen/html/ArgumentPromotion_8cpp_source.html#l00344
Given an Argument (in your case %a or %b), it will first get a pointer to the Function in Callee (for you it is foo()), and then loop over the users of the function which it expects to be of type CallSite.
