Extended Description

Consider this C++ testcase:

#include
void test1(int &x, int &y) {
x = std::min(x,y);
}

void test2(int &x, int &y) {
int tmp = x;
y = std::max(tmp,y);
}

We're currently compiling this into:

void %Z5test1RiS(int* %x, int* %y) {
entry:
%tmp.1.i = load int* %y
%tmp.3.i = load int* %x
%tmp.4.i = setlt int %tmp.1.i, %tmp.3.i
%retval.i = select bool %tmp.4.i, int* %y, int* %x
%tmp.4 = load int* %retval.i
store int %tmp.4, int* %x
ret void
}

Note that for this function the %tmp.4 load is redundant with the loads of X or
Y. It would be better to select one of those load's values.

void %Z5test2RiS(int* %x, int* %y) {
entry:
%tmp.0 = alloca int
%tmp.2 = load int* %x
store int %tmp.2, int* %tmp.0
%tmp.3.i = load int* %y
%tmp.4.i = setlt int %tmp.2, %tmp.3.i
%retval.i = select bool %tmp.4.i, int* %y, int* %tmp.0
%tmp.6 = load int* %retval.i
store int %tmp.6, int* %y
ret void
}

This is even worse. Here we are doing the same thing, selecting the address of
Y or the tmp. Because we need it's address, we are not able to register promote
the tmp, leaving it as an alloca. :(

We should transform these selects into selects of the value not of the pointer
when safe.

-Chris