[fpc-devel] x86-64 compilation of while loops

[J. Gareth Moreton]

 I've come across an interesting situation with the compiler when it comes
to while loops.  It's not necessary erroneous, but I wonder if it incurs a
speed penalty.

 Using my code sample to demonstrate:

 ****

 type
   TBinList = record
     Key: Integer;
     Data: CodePointer;
   end;
   PBinList = ^TBinList;

 function SearchBinList(List: PBinList; ListSize: Cardinal; Key: Integer;
out Data: CodePointer): Boolean;
 var
   LoIndex, MidIndex, HiIndex: Cardinal;
 begin
   LoIndex := 0;
   HiIndex := ListSize;

   { Search binary list }
   while LoIndex + 1 < HiIndex do
     begin
       MidIndex := (LoIndex + HiIndex) shr 1;

       if List[MidIndex].Key > Key then
         HiIndex := MidIndex
       else
         LoIndex := MidIndex;

     end;

   Data := List[LoIndex].Data;
   Result := (List[LoIndex].Key = Key);
 end;

 ****

 This is a binary search algorithm that I've streamlined as much as I can
for speed (e.g. there's no sanity check for ListSize being zero, and it
doesn't immediately exit the loop if it finds a match.

 The generated assembly (-O3) for the while loop is as follows:

 ****
     xorl    %r10d,%r10d
     jmp    .Lj6
     .balign 8,0x90
 .Lj5:
     movl    %r10d,%eax
     movl    %edx,%r11d
     addq    %r11,%rax
     shrq    $1,%rax
     movl    %eax,%ebx
     andl    $4294967295,%eax
     shlq    $4,%rax
     cmpl    (%rcx,%rax),%r8d
     cmovll    %ebx,%edx
     cmovnll    %ebx,%r10d
 .Lj6:
     movl    %r10d,%eax
     leaq    1(%rax),%r11
     movl    %edx,%eax
     cmpq    %rax,%r11
     jl    .Lj5
     ...
 ****

 In this case, the ".balign" hint adds 2 bytes to pad the loop.  However,
my question is this... why does it immediately jump to the end of the loop
to check the condition (which is very likely to be true on the first
iteration), only to jump to the beginning again? Granted, in this case the
condition is relatively simple, but why not simply check the condition
where the "jmp" instruction is, and jumping to the line after the "jl"
instruction if it's false? For example:

     xorl    %r10d,%r10d // 
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