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Aleksey Shipilev commented on HARMONY5901:

That's good, Xiaoming!
A couple of little "polishing" comments:
a. eliminate "&& true" from if conditions, that was introduced by me for debugging purposes and I forgot to sweep them out
b. move out common blocks from switch cases
After that the patch can be committed, if it passes VM/Classlib tests.
> replace integer multiplication or division with shift if one of multipliers or divisor is a power of 2
> 
>
> Key: HARMONY5901
> URL: https://issues.apache.org/jira/browse/HARMONY5901
> Project: Harmony
> Issue Type: Improvement
> Components: DRLVM
> Reporter: Xiaoming Gu
> Attachments: 5901.patch, H5901V2.patch, H5901V3.patch, H5901V4.patch
>
>
> 1. Mulitiplication replacement
> a. It's about integer multiplication.
> b. One of two multipliers is a power of 2.
> c. The power of 2 could be negative.
> For example, 23*4 is transformed to 23<<2 and 23*(4) is transformed to (23<<2)*(1).
> 2. Division replacement
> a. It's about integer division.
> b. The divisor is a power of 2.
> c. The power of 2 could be negative.
> d. Because of roundup difference between division and shift right, the following equations from Aleksey are used.
> Baseline: q = a / (1 << d);
> Optimized: q = (a + [(1 << d)  1] & (a >> 31)) >> d;
> For example, 23/4 is transformed to (23+((1<<2)1)&(23>>31))>>2. and (23)/(4) is transformed to ((23+((1<<2)1)&(23>>31))>>2)*(1).
> 3. Comments for SAR and SHR swapped
> The comments for the two similar operations were misplaced in working_vm/vm/port/src/encoder/ia32_em64t.

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