[llvm] [RISCV][MRI] Account for fixed registers when determining callee saved regs (PR #115756)

[Craig Topper via llvm-commits]

topperc wrote:

> > Does this change the stack layout of callee saved registers when one is reserved?
> 
> This patch causes us not to spill the reserved register, when we were spilling it before. This by definition changes the stack layout.
> 
> > Does it match gcc stack layout?
> 
> I tried with this program with my patch applied:
> 
> ```
> register long a asm("x24");
> int bar();
> int baz(int a, int b, int c, int d);
> int foo(int b, int c, int d) {
>   a = 123;
>   bar();
>   baz(a, b, c, d);
>   return 0;
> }
> ```
> 
> GCC:
> 
> ```
> sd      ra,24(sp)
> sd      s0,16(sp)
> sd      s1,8(sp)
> sd      s2,0(sp)
> ```
> 
> LLVM + this patch:
> 
> ```
> sd ra, 24(sp) # 8-byte Folded Spill
> sd s0, 16(sp) # 8-byte Folded Spill
> sd s1, 8(sp) # 8-byte Folded Spill
> sd s2, 0(sp) # 8-byte Folded Spill
> ```
> 
> Looks the same on this example.

A better test would spill s7 and s9, but not s8. I want to know if s8 still gets allocated a slot in gcc.

https://github.com/llvm/llvm-project/pull/115756