RFR: 8312213: Remove unnecessary TEST instructions on x86 when flags reg will already be set

[Quan Anh Mai]

On Fri, 26 May 2023 10:32:00 GMT, Tobias Hotz <duke at openjdk.org> wrote:

> This patch adds peephole rules to remove TEST instructions that operate on the result and right after a AND, XOR or OR instruction.
> This pattern can emerge if the result of the and is compared against two values where one of the values is zero. The matcher does not have the capability to know that the instruction mentioned above also set the flag register to the same value.
> According to https://www.felixcloutier.com/x86/and, https://www.felixcloutier.com/x86/xor, https://www.felixcloutier.com/x86/or and https://www.felixcloutier.com/x86/test the flags are set to same values for TEST, AND, XOR and OR, so this should be safe.
> By adding peephole rules to remove the TEST instructions, the resulting assembly code can be shortend and a small speedup can be observed:
> Results on Intel Core i5-8250U CPU
> Before this patch:
> 
> Benchmark                                              Mode  Cnt    Score    Error  Units
> TestRemovalPeephole.benchmarkAndTestFusableInt         avgt    8  182.353 ±  1.751  ns/op
> TestRemovalPeephole.benchmarkAndTestFusableIntSingle   avgt    8    1.110 ±  0.002  ns/op
> TestRemovalPeephole.benchmarkAndTestFusableLong        avgt    8  212.836 ±  0.310  ns/op
> TestRemovalPeephole.benchmarkAndTestFusableLongSingle  avgt    8    2.072 ±  0.002  ns/op
> TestRemovalPeephole.benchmarkOrTestFusableInt          avgt    8   72.052 ±  0.215  ns/op
> TestRemovalPeephole.benchmarkOrTestFusableIntSingle    avgt    8    1.406 ±  0.002  ns/op
> TestRemovalPeephole.benchmarkOrTestFusableLong         avgt    8  113.396 ±  0.666  ns/op
> TestRemovalPeephole.benchmarkOrTestFusableLongSingle   avgt    8    1.183 ±  0.001  ns/op
> TestRemovalPeephole.benchmarkXorTestFusableInt         avgt    8   88.683 ±  2.034  ns/op
> TestRemovalPeephole.benchmarkXorTestFusableIntSingle   avgt    8    1.406 ±  0.002  ns/op
> TestRemovalPeephole.benchmarkXorTestFusableLong        avgt    8  113.271 ±  0.602  ns/op
> TestRemovalPeephole.benchmarkXorTestFusableLongSingle  avgt    8    1.183 ±  0.001  ns/op
> 
> After this patch:
> 
> Benchmark                                              Mode  Cnt    Score   Error  Units  Change
> TestRemovalPeephole.benchmarkAndTestFusableInt         avgt    8  141.615 ± 4.747  ns/op  ~29% faster
> TestRemovalPeephole.benchmarkAndTestFusableIntSingle   avgt    8    1.110 ± 0.002  ns/op  (unchanged)
> TestRemovalPeephole.benchmarkAndTestFusableLong        avgt    8  213.249 ± 1.094  ns/op  (unchanged)
> TestRemovalPeephole.benchmarkAndTestFusableLongSingle  avgt    8    2.074 ± 0.011...

In general I think this approach is very elegant. A point I want to address is the definition of setting a flag, a `test` can be elided after an `and` because the `and` sets the flag according to **the result**, instructions can set flags based on other factors, such as `add` sets OF based on the operation, `bsr` sets ZF based on the input, etc. As a result, I think for those that does not set the flags based on the result, it should be marked separately, and anything else should be marked as cloberring.

Thanks.

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PR Comment: https://git.openjdk.org/jdk/pull/14172#issuecomment-1661873540