On Tue, Jul 04, 2017 at 05:11:55PM -0700, Walter Bright via 
Digitalmars-d-announce wrote:
> On 7/4/2017 4:14 PM, H. S. Teoh via Digitalmars-d-announce wrote:
> > Also, loop unrolling is only the beginning.  Other loop
> > optimizations are just as important, like strength reduction,
> > hoisting, etc.. (Caveat: I haven't checked whether DMD specifically
> > performs these optimizations.
> It does.
> > But based on looking at previous dmd output, I'm leaning towards
> > no.)
> I wish people would look at it before assuming. It's not like it's a
> secret.
>   https://github.com/dlang/dmd/blob/master/src/ddmd/backend/gloop.c
> Read the comments.

I did a simple test to see which loop optimizations dmd did, vs. gdc.
Here's the test code:

int func(int[] data)
        int i, j;
        for (i = 0; i < 10; i++) {
                data[i*10] = i;
                j = data[0] * 10;
        return j;
void main() {
        import std.stdio;
        auto data = new int[100];

Here's the output of dmd -O (git HEAD):

0000000000046b00 <_D4test4funcFAiZi>:
   46b00:       55                      push   %rbp
   46b01:       48 8b ec                mov    %rsp,%rbp
   46b04:       48 89 fa                mov    %rdi,%rdx
   46b07:       49 89 f1                mov    %rsi,%r9
   46b0a:       45 31 c0                xor    %r8d,%r8d
   46b0d:       31 c9                   xor    %ecx,%ecx
   46b0f:       48 63 c1                movslq %ecx,%rax
   46b12:       48 3b c2                cmp    %rdx,%rax
   46b15:       72 11                   jb     46b28 <_D4test4funcFAiZi+0x28>
   46b17:       be 05 00 00 00          mov    $0x5,%esi
   46b1c:       48 8d 3d cd 53 03 00    lea    0x353cd(%rip),%rdi        # 
7bef0 <_TMP0>
   46b23:       e8 64 0a 00 00          callq  4758c <_d_arrayboundsp>
   46b28:       45 89 04 81             mov    %r8d,(%r9,%rax,4)
   46b2c:       48 85 d2                test   %rdx,%rdx
   46b2f:       75 11                   jne    46b42 <_D4test4funcFAiZi+0x42>
   46b31:       be 06 00 00 00          mov    $0x6,%esi
   46b36:       48 8d 3d b3 53 03 00    lea    0x353b3(%rip),%rdi        # 
7bef0 <_TMP0>
   46b3d:       e8 4a 0a 00 00          callq  4758c <_d_arrayboundsp>
   46b42:       41 8b 01                mov    (%r9),%eax
   46b45:       44 8d 1c 80             lea    (%rax,%rax,4),%r11d
   46b49:       45 03 db                add    %r11d,%r11d
   46b4c:       83 c1 0a                add    $0xa,%ecx
   46b4f:       41 ff c0                inc    %r8d
   46b52:       41 83 f8 0a             cmp    $0xa,%r8d
   46b56:       72 b7                   jb     46b0f <_D4test4funcFAiZi+0xf>
   46b58:       41 8b c3                mov    %r11d,%eax
   46b5b:       5d                      pop    %rbp
   46b5c:       c3                      retq   
   46b5d:       00 00                   add    %al,(%rax)

Note: for conciseness' sake I omitted the disassembly of main(), since
it's not directly relevant here.

Here are some pertinent points of observation:

- Strength reduction was done, as seen in the line 46b4c: corresponding
  with the array index computation i*10.

- Code hoisting was NOT done (in this case): the second line in the loop
  body does not depend on the loop index, but dmd did not hoist it out
  of the loop. This can be see by the end of loop branch on line 46b56:
  the branch destination is 46b0f, near the beginning of the function,
  and the code path from there includes the code for the assignment to
  j. While some clever tricks were done to avoid using the mul
  instruction for computing data[0]*10, this computation was
  unfortunately repeated 10 times even though it only needed to be
  computed once. In particular, the load of data[0] on line 46b42 is
  repeated 10 times, followed by the *10 computation.

- There are two calls to _d_arrayboundsp inside the loop body, along
  with branches around them. This seems needless, since one bounds check
  ought to be enough to ensure the array lookups are within bounds.
  Also, there are 2 branches within the loop body (not counting the
  end-of-loop branch), whereas it could have been simplified to one
  (less branch hazards on the CPU pipeline).

In comparison, here's the output of gdc -O (gdc 6.3.0):

0000000000020080 <_D4test4funcFAiZi>:
   20080:       48 85 ff                test   %rdi,%rdi
   20083:       74 33                   je     200b8 <_D4test4funcFAiZi+0x38>
   20085:       48 89 f9                mov    %rdi,%rcx
   20088:       49 89 f0                mov    %rsi,%r8
   2008b:       c7 06 00 00 00 00       movl   $0x0,(%rsi)
   20091:       ba 0a 00 00 00          mov    $0xa,%edx
   20096:       b8 01 00 00 00          mov    $0x1,%eax
   2009b:       48 39 d1                cmp    %rdx,%rcx
   2009e:       76 18                   jbe    200b8 <_D4test4funcFAiZi+0x38>
   200a0:       41 89 04 90             mov    %eax,(%r8,%rdx,4)
   200a4:       83 c0 01                add    $0x1,%eax
   200a7:       48 83 c2 0a             add    $0xa,%rdx
   200ab:       83 f8 0a                cmp    $0xa,%eax
   200ae:       75 eb                   jne    2009b <_D4test4funcFAiZi+0x1b>
   200b0:       8b 06                   mov    (%rsi),%eax
   200b2:       8d 04 80                lea    (%rax,%rax,4),%eax
   200b5:       01 c0                   add    %eax,%eax
   200b7:       c3                      retq   
   200b8:       48 83 ec 08             sub    $0x8,%rsp
   200bc:       ba 05 00 00 00          mov    $0x5,%edx
   200c1:       bf 06 00 00 00          mov    $0x6,%edi
   200c6:       48 8d 35 81 7a 07 00    lea    0x77a81(%rip),%rsi        # 
97b4e <_IO_stdin_used+0x4e>
   200cd:       e8 8e a4 03 00          callq  5a560 <_d_arraybounds>

Comparing this with dmd's output, we see:

- Strength reduction was done on the i*10 computation (line 200a7), just
  as in the dmd output.

- Code hoisting was also done (unlike dmd): the computation data[0]*10 was
  hoisted out of the loop (line 200b2), and only computed once after the
  end of the loop, as opposed to computed 10 times. Notably, we're no
  longer loading data[0] 10 times, but just once at the end of the loop.

- One of the bounds checks is moved out of the loop body, so there is
  only 1 branch inside the loop (less branch hazards on the CPU

- The function is noticeably smaller than dmd's output, due to gdc
  merging the calls to _d_arraybounds into a single path.

Now, granted, my test case could be construed to be unfair, because the
assignment to j depends on the result of the first loop iteration
(data[0] is assigned to before it's read by the assignment to j). So
it's not truly loop-invariant in the strict sense.  However, as the gdc
output shows, the compiler ought to be able to refactor things so that
the assignment is moved out of the loop.

So while I was wrong about dmd not doing strength reduction, my
conclusion is still that dmd's codegen for loops leaves more to be
desired.  In particular, it doesn't seem to do code hoisting, as least
not for this case, whereas gdc does (and consistently so in other loop
code I've looked at in the past).


An imaginary friend squared is a real enemy.

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