[CORRECTION] [compiler-rt] _udivdi3(), _umoddi3(), _moddi3() and _divdi3() routines not properly "tuned"

[Friedman, Eli via llvm-commits]

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-Eli

On 11/9/2017 1:33 AM, Stefan Kanthak via llvm-commits wrote:
> Hi,
>
> replace the proposed fix #3 from my initial post with the following
> faster version:
>
>> Bug #3: such  "highly tuned" routines should but come without large
>> ~~~~~~~ duplicate code sequences.
>>
>> In the 4 routines named in the subject, the code from label 1: to the
>> respective return is almost identical to the code preceeding label 1:;
>> the only difference is the initial subtraction and the insertion of
>> a leading 1 into the quotient.
>>
>> Fix #3: remove all lines between "jae 1f" (including the wrong
>> ~~~~~~~ comment which follows "jae 1f") and the label 1:, then
>>          apply the following diff (yes, this adds one or two
>>          instructions to the overall execution path, but should
>>          typically cost no cycles, since they can execute in parallel).
> +   pushl %edi
> +   xorl  %edi, %edi // MSB of quotient
>      cmpl  %ebx, %edx // to avoid overflowing the upcoming divide.
> +   jb    0f
> -   jae   1f
>
> 1: /* High word of a is greater than or equal to (b >> (1 + i)) on this branch */
>
> +   movl  $0x80000000, %edi       // MSB of quotient
>      subl  %ebx, %edx // subtract bhi from ahi so that divide will not
> +
> +0: /* High word of a is smaller than (b >> (1 + i)) on this branch */
> +
>      divl %ebx // overflow, and find q and r such that
>      //
>      // ahi:alo = (1:q)*bhi + r
>      //
>      // Note that q is a number in (31-i).(1+i)
>      // fix point.
> -   pushl %edi
>      notl  %ecx
>      shrl  %eax
> +   orl   %eax       // insert proper MSB into quotient
> -   orl   $0x80000000, %eax
>
> regards
> Stefan Kanthak
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