RFR: 8257815: Replace global log2 functions with efficient implementations [v8]

[Stefan Karlsson]

On Wed, 9 Dec 2020 18:26:10 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> This patch replaces the log2 functions in globalDefinitions.hpp with more efficient counterparts in utilities/powerOfTwo.hpp
>> 
>> Naming is hard, but I think the following scheme is reasonable:
>> 
>> - log2i: any integral type. 0-hostile
>> - log2i_allow_zero: any integral type. gracefully handles zero (adds a branch)
>> - exact_log2i: any integral type. value must be a power of two
>> 
>> I chose log2i rather than log2 to stand out from the log2 functions defined in various standard libraries.
>> 
>> Going through all usage, quite a few uses of log2_long et.c. could be replaced by exact_log2i since they take something that has been checked to be a power of two. Most of the remaining usage seem to be able to use the 0-hostile variant, which avoids a branch.
>> 
>> To sanity check that calculating log2 using count_leading_zeros gives better performance I added a couple of trivial and short-running microbenchmarks to test_powerOfTwo. For small values (<= 1025) the new impl is ~5x faster, with a larger speed-up for larger integer values:
>> 
>> [ RUN      ] power_of_2.log2_long_micro
>> [       OK ] power_of_2.log2_long_micro (3581 ms)
>> [ RUN      ] power_of_2.log2_long_small_micro
>> [       OK ] power_of_2.log2_long_small_micro (549 ms)
>> [ RUN      ] power_of_2.log2i_micro
>> [       OK ] power_of_2.log2i_micro (259 ms)
>> [ RUN      ] power_of_2.log2i_small_micro
>> [       OK ] power_of_2.log2i_small_micro (113 ms)
>> 
>> I'm not sure if this naive microbenchmark carries its own weight, but it just adds a few seconds and can be useful for quickly checking this performance assumption on other H/W
>> 
>> (Intending this for 17)
>
> Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Issue with using count_trailing_zeros with 64-bit values on 32-bit, revert and add TODO

I'm fine with the change as is. I have a few comments that you might want to consider.

src/hotspot/share/utilities/powerOfTwo.hpp line 79:

> 77:                        (uint64_t)static_cast<typename std::make_unsigned<T>::type>(value));
> 78:   // TODO: this could be "return count_trailing_zeros(value)"", but that fails on
> 79:   // 32-bit builds since that takes uintx and is thus lossy for 64-bit values.

I don't think we need to leave this bread crumbs in the code. If there's really something we want "to do", then we should create an RFE for it instead.

src/hotspot/share/gc/z/zHeuristics.cpp line 48:

> 46:     // Enable medium pages
> 47:     ZPageSizeMedium             = size;
> 48:     ZPageSizeMediumShift        = log2i(ZPageSizeMedium);

I think you missed the comment that this could be log2i_exact.

src/hotspot/cpu/x86/vm_version_x86.cpp line 784:

> 782:               cpu_family(), _model, _stepping, os::cpu_microcode_revision());
> 783:   assert(res > 0, "not enough temporary space allocated");
> 784:   assert(log2i_exact((uint64_t)CPU_MAX_FEATURE) + 1 == sizeof(_features_names) / sizeof(char*), "wrong size features_names");

If you want to get rid of this cast (and the one added to interp_masm_x86.cpp, you could register your log2i functions to accept enums:
-template <typename T, ENABLE_IF(std::is_integral<T>::value)>
+template <typename T, ENABLE_IF(std::is_integral<T>::value || std::is_enum<T>::value)>

test/hotspot/gtest/utilities/test_powerOfTwo.cpp line 276:

> 274:   }
> 275:   {
> 276:     /* Test log2i_graceful handles 0 input */

Could convert /* */ to //

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Marked as reviewed by stefank (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1663