[Issue 20189] New: Codegen - functions that call other functions with the same arguments do redundant copying between stack and registers.

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20189

          Issue ID: 20189
           Summary: Codegen - functions that call other functions with the
                    same arguments do redundant copying between stack and
                    registers.
           Product: D
           Version: D2
          Hardware: x86_64
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: elronnd at elronnd.net

Consider this code - contrived, but representative of an actual problem I had:

    pragma(inline, false) int second(int a, int b) {
            return a+b;
    }
    pragma(inline, false) int first(int a, int b) {
            return second(a, b);
    }

It generates this code for first() with dmd version 2.087.1, with optimizations
enabled:

push   rbp
mov    rbp,rsp
sub    rsp,0x10
mov    DWORD PTR [rbp-0x10],edi
mov    DWORD PTR [rbp-0x8],esi
mov    esi,DWORD PTR [rbp-0x8]
mov    edi,DWORD PTR [rbp-0x10]
call   3c66c <int perftest.second(int, int)>
mov    rsp,rbp
pop    rbp
ret

It moves both of its parameters--which are in the edi and esi registers--onto
the stack, and then...back into the registers.  Those 4 movs in the middle
could be elided completely.

(As a further optimization, 'return func(...)' could be translated into 'jmp
func', instead of 'call func; ret'--ldc does this.)

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