llvm dev - Jun 2013 - [LLVMdev] ldexp constant propagation

Hi,

I have the C program below,  which contains a mathematical expression.

C code:
=====void ControlledRotationPiMinus(int target, int control, int j)
{
	ControlledPhase(target,control,-PI/(2*pow(2,j)));
}

int main() {
	..
	int b = 3;
	for (int j=1; j < b; j++) 
		ControlledRotationPiMinus(target, control, j);
	..
}

Here is what I am doing: The function ControlledRotationPiMinus is called twice
in main(), with values of j=1,2. I do procedure cloning (using
CloneAndPruneFunctionInto) resulting in 2 copies of this function with the
values of j set to a fixed number (1 and 2).
However the mathematical expression inside these copies fails to resolve to a
fixed number and propagate. So for the first copy, with j=1, I get the following
llvm code:

LLVM code:
========
define void @ControlledRotationPiMinus_1(i32 %target, i32 %control, i32 %j) {
entry.:
  %exp2. = call double @ldexp(double 1.000000e+00, i32 1) nounwind
  %mul. = fmul double %exp2., 2.000000e+00
  %div. = fdiv double -3.141590e+00, %mul.
  call void @ControlledPhase(i32 %target, i32 %control, double %div.)
  ret void
}

I want to get a constant 3rd argument in the last call instruction
(ControlledPhase), but it does not happen (I tried a lot of things including
-constprop, -instcombine, etc.).
I found the problem to be with "ldexp". If I change my mathematical
expression to something else (e.g. containing "sin", "sqrt",
etc.) the constant propagation does happen. Any ideas on why this doesn't
work for exponentials, and how I can fix it?

Thanks,
Ali

Hi Ali,
> I found the problem to be with "ldexp". If I change my
mathematical expression to something else (e.g. containing "sin",
"sqrt", etc.) the constant propagation does happen. Any ideas on why
this doesn't work for exponentials, and how I can fix it?
I think the code that's supposed to do what you want is in
lib/Analysis/ConstantFolding.cpp, which indeed doesn't know how to
handle ldexp yet. Unfortunately there's no ldexp intrinsic yet either
so you'll have to plumb that through LLVM too.

Tim