RFR: 8308465: Reduce memory accesses in AArch64 MD5 intrinsic [v2]

[Paul Hohensee]

On Sat, 20 May 2023 20:11:52 GMT, Yi-Fan Tsai <duke at openjdk.org> wrote:

>> Two optimizations have been implemented in this change to reduce memory reads in AArch64 MD5 intrinsic.
>> 
>> **Optimization 1:** Memory loads and stores updating hash values are moved out of the loop. The final results are only written to memory once.
>> 
>> The original snippet loaded the value (step 3) soon after it was written to the memory (step 2). 
>> 
>> md5_loop:
>>     __ ldrw(a, Address(state, 0));         // step 3: load the value from memory
>>     ... // loop body
>>     __ ldrw(rscratch1, Address(state, 0)); // step 1: load the value at Address(state, 0)
>>     __ addw(rscratch1, rscratch1, a);
>>     __ strw(rscratch1, Address(state, 0)); // step 2: write the value to memory
>>     ...
>>     __ br(Assembler::LE, md5_loop);
>> 
>> 
>> The snippet is optimized to avoid memory loads and writes in the loop.
>> 
>>     __ ldp(s0, s1, Address(state,  0));    // load the value at Address(state, 0) to a register
>>     __ ubfx(a, s0, 0, 32);
>> md5_loop:
>>     .. // body
>>     __ ubfx(rscratch1, s0, 0, 32);         // step 1: extract the value from the register
>>     __ addw(a, rscratch1, a);
>>     __ orr(s0, a, b, Assembler::LSL, 32);  // step 2: preserve the value in the register
>>     ....
>>     __ br(Assembler::LE, md5_loop);
>>     ....
>>     __ str(s0, Address(state, 0));         // write the result to memory only once
>> 
>> 
>> **Optimization 2**: Redundant loads generated by `md5_GG`, `md5_HH`, and `md5_II` are removed.
>> 
>> The original snippet, generated by two `md5_FF`s and `md5_GG`s, read the same data repeatedly.
>> 
>> __ ldrw(rscratch1, Address(buf, 0));    // from md5_FF(.., k = 0, ..)
>> ...
>> __ ldrw(rscratch1, Address(buf, 4));    // from md5_FF(.., k = 1, ..)
>> ...
>> __ ldrw(rscratch1, Address(buf, 4));    // from md5_GG(.., k = 1, ..)
>> ...
>> __ ldrw(rscratch1, Address(buf, 0));    // from md5_GG(.., k = 0, ..)
>> 
>> 
>> The snippet is optimized by caching the values in registers and removing the redundant loads.
>> 
>> __ ldp (buf0, buf1, Address(buf, 0));  // load both values into buf0
>> ...
>> __ ubfx(rscratch1, buf0, 0, 32);       // extract the value of k = 0 from the lower 32 bits of buf0
>> ...
>> __ ubfx(rscratch1, buf0, 32, 32);      // extract the value of k = 1 from the higher 32 bits of buf0
>> ...
>> __ ubfx(rscratch1, buf0, 32, 32); 
>> ...
>> __ ubfx(rscratch1, buf0, 0, 32);
>> 
>> 
>> 
>> **Test**
>> The following tests have passed.
>> 
>> test/hotspot/jtreg/compiler/intrinsics/sha/sanity/TestMD5Intrinsics.java
>> test/hotspot/jtreg/compiler/intrinsics...
>
> Yi-Fan Tsai has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Rename and optimize

Marked as reviewed by phh (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/14068#pullrequestreview-1437042848