Re: MPS prstack

[Helmut Eller]

On Mon, May 27 2024, Gerd Möllmann wrote:

>>   prstack.stack[prstack.sp++] = e;
>
> Hm. No idea when the increment is sequenced. The side effect of postfix
> ++ could be applied anywhere before the next sequence point. But where
> is that sequence point? There is one at the end of the whole expression
> I think.

The generated code with O0 looks like this:

   ...
   0x000055555587fcb1 <+28>:    call   0x55555587fbd0 <grow_print_stack>
   0x000055555587fcb6 <+33>:    mov    0x8845c3(%rip),%rcx        # 
0x555556104280 <prstack>
   0x000055555587fcbd <+40>:    mov    0x8845cc(%rip),%rax        # 
0x555556104290 <prstack+16>
   0x000055555587fcc4 <+47>:    lea    0x1(%rax),%rdx
   0x000055555587fcc8 <+51>:    mov    %rdx,0x8845c1(%rip)        # 
0x555556104290 <prstack+16>
   0x000055555587fccf <+58>:    mov    %rax,%rdx
   0x000055555587fcd2 <+61>:    mov    %rdx,%rax
   0x000055555587fcd5 <+64>:    shl    $0x3,%rax
   0x000055555587fcd9 <+68>:    sub    %rdx,%rax
   0x000055555587fcdc <+71>:    shl    $0x3,%rax
   0x000055555587fce0 <+75>:    add    %rcx,%rax
   0x000055555587fce3 <+78>:    mov    0x10(%rbp),%rcx
   0x000055555587fce7 <+82>:    mov    0x18(%rbp),%rbx
   ...

So it looks like prstack.sp is updated with the move at +51, i.e. before
the stack slot is initialized.

We could write

  prstack.stack[prstack.sp] = e;
  prstack.sp += 1;

Not sure how we then guarantee that e still in a register or the control
stack at the second line.

But my question was more whether MPS actually does call scan_prstack at
"any moment" or if this is just a theoretical possibility for some
possible future version of MPS.