[llvm-dev] Register spilling in caller saved backend

[Peter Bergner via llvm-dev]

On Tue, 2015-09-01 at 08:52 -0700, Jacques Pienaar via llvm-dev wrote:

> int main() {
>   for (int i = 0; i < 10; ++i) {
>     foo();
>   }
> }
> 
> 
> The resultant output code spills 'i' within the loop (I have to
> compile with -fno-unroll-loops else the loop and 'i' gets optimi! zed
> away) while 'i' is assigned to a register that is used nowhere else in
> the generated program and I would consider 'foo' easy to analyse. At
> the call site of 'foo' the debugging output reports that the stack
> pointer and return value registers are "imp-use" and "imp-def" and no
> other registers are used.

When a function (such as main() in this case) is compiled, all calls to
functions are assumed to clobber all caller saved registers.  Since your
backend said that all registers are caller saved, then the compiler has
not choice but to spill 'i' around the call to foo(), since it may
trash the register 'i' is allocated to.


> Should the spilling have occurred? Or this due to some feature in the
> backend that is missing?

This is totally as expected when you have no callee saved registers.


Peter