[Issue 14943] dmd should inline more aggressively

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=14943

--- Comment #1 from hsteoh at quickfur.ath.cx ---
Further notes:
- gdc not only inlines the call trees of .empty, .front, .popFront, it also
applied other loop optimizations like strength reduction to refactor the a =>
a*7 into a += b; b += 7. Not sure if dmd is capable of doing this, but in any
case the opportunity is missed because .popFront was not inlined, so the
optimizer wouldn't have been able to apply strength reduction.

- gdc's aggressive inlining also allowed various loop counters and accumulators
to be completely held in registers, while the function calls generated by dmd
necessitated dereferencing addresses to stack variables, which is an extra
layer of indirection. Again, a missed opportunity due to not inlining
aggressively enough.

For reference, here's the inner loop produced by gdc:

  403b80:       89 d7                   mov    %edx,%edi
  403b82:       c1 ef 1f                shr    $0x1f,%edi
  403b85:       8d 34 3a                lea    (%rdx,%rdi,1),%esi
  403b88:       83 c2 07                add    $0x7,%edx
  403b8b:       83 e6 01                and    $0x1,%esi
  403b8e:       39 fe                   cmp    %edi,%esi
  403b90:       75 1e                   jne    403bb0 <int test.fun(int)+0x80>
  403b92:       89 c6                   mov    %eax,%esi
  403b94:       8d 14 cd 00 00 00 00    lea    0x0(,%rcx,8),%edx
  403b9b:       c1 ee 1f                shr    $0x1f,%esi
  403b9e:       01 f0                   add    %esi,%eax
  403ba0:       29 ca                   sub    %ecx,%edx
  403ba2:       d1 f8                   sar    %eax
  403ba4:       01 d0                   add    %edx,%eax
  403ba6:       83 c2 07                add    $0x7,%edx
  403ba9:       0f 1f 80 00 00 00 00    nopl   0x0(%rax)
  403bb0:       83 c1 01                add    $0x1,%ecx
  403bb3:       39 cb                   cmp    %ecx,%ebx
  403bb5:       75 c9                   jne    403b80 <int test.fun(int)+0x50>

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