Q: C++ FE emitting assignments to global read-only symbols?

[Diego Novillo]

One of the micro-optimizations I am about to merge from TCB
involves disregarding V_MAY_DEF/V_MUST_DEF operands for read-only
globals.

So, if a symbol is marked read-only and the operand scanner
requests a V_MAY_DEF or V_MUST_DEF operand for it, we replace it
with a VUSE.

This works fine, except that I was having comparison errors with
SPEC2000's eon.  I tracked it down to the C++ FE emitting an
assignment instruction for a global const variable.  What follows
is speculation, I have no idea how the FE really works:

The source code declares

const double ggPi = 3.14159265358979323846;
double const divPi = 1 / ggPi;

And since divPi is initialized to an expression, I guess it needs
to compute it at runtime, so it emits an initialization function:

void __static_initialization_and_destruction_0(int, int) (__initialize_p, 
__priority)
{
  ...
  if (D.55019)
    {
      ggPi.319 = ggPi;
      D.55021 = 1.0e+0 / ggPi.319;
      divPi = D.55021;
    }
  ...
}

So, we now have an assignment for divPi in the IL stream.  This
throws a monkey wrench into my micro-optimization because it
shouldn't really have ignored that V_MUST_DEF (we end up removing
the assignment).

I also wouldn't want the FE to mark divPi writeable, as we would
now consider it call-clobbered.  Is this analysis correct?  Will
other FEs need to do things like this?  If so, then I'll just get
rid of this transformation.  We are already ignoring read-only
symbols at call sites and there are new analysis for trimming
clobbering lists even further down the pipe.


Thanks.  Diego.