RFR: 8324720: Instruction selection does not respect -XX:-UseBMI2Instructions flag

[Roberto Castañeda Lozano]

On Fri, 23 May 2025 13:50:09 GMT, Saranya Natarajan <duke at openjdk.org> wrote:

> While executing a function  performing `a >> b` operation with `–XX:-UseBMI2Instructions` flag, the generated code contains BMI2 instruction `sarx eax,esi,edx`. The expected output should not contain any BMI2 instruction.  
>  
> ### Analysis and solution 
>  
> As suggested by @merykitty in [JDK-8324720](https://bugs.openjdk.org/browse/JDK-8324720) , the initial idea was to make `VM_Version::supports_bmi2()` respect` UseBMI2Instructions `flag by disabling BMI2 feature when `UseBMI2Instructions` runtime flag is explicitly set to false. This fix is similar to how other runtime flags such as, `UseAPX` and `UseAVX`, enable or disable specific code and register set. However, some test failures were encountered while running tests on this fix. 
> 
> The first set of failures were caused by assertion check on `VM_Version::supports_bmi2()` statement while generating some BMI2 specific instructions. This was caused by the stub generator generating AVX-512 specific code that uses these BMI2 instructions. It should be noted that the `UseAVX` flag is set by default to the highest supported version available in x86 machine. This in turn allows AVX-512 specific code generation whenever possible. In order to not comprise the performance benefits of using AVX-512,  the proposed fix only disables BMI2 feature if AVX-512 features are also disabled  (or not available in the machine) along with the UseBMI2Instructions flag.  
>  
> The second failure occured in `compiler/intrinsics/sha/cli/TestUseSHA512IntrinsicsOptionOnSupportedCPU.java` where a warning "_Intrinsics for SHA-384 and SHA-512 crypto hash functions not available on this CPU_." was returned on a AMD64 machine that had support for SHA512. Looking into `compiler/testlibrary/sha/predicate/IntrinsicPredicates.java` it was found that the predicate for AMD64 was not in line with the changes introduced by [JDK-8341052](https://bugs.openjdk.org/browse/JDK-8341052) in commit [85c1aea](https://github.com/openjdk/jdk/pull/20633/commits/85c1aea90b10014aa34dfc902dff2bfd31bd70c0) .

This change affects the final value of `UseBMI2Instructions` when the JVM is run with its default configuration on machines without AVX-512 (i.e. `UseAVX` <= 2), I guess that is unexpected?

My CPU flags:

$ cat /proc/cpuinfo | grep flags
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb ssbd ibrs ibpb stibp ibrs_enhanced tpr_shadow flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid mpx rdseed adx smap clflushopt intel_pt xsaveopt xsavec xgetbv1 xsaves dtherm ida arat pln pts hwp hwp_notify hwp_act_window hwp_epp vnmi md_clear flush_l1d arch_capabilities


Before this change:

$ java -XX:+PrintFlagsFinal | grep UseBMI
     bool UseBMI1Instructions                      = true                                 {ARCH product} {default}
     bool UseBMI2Instructions                      = true                                 {ARCH product} {default}

With this change:

$ java -XX:+PrintFlagsFinal --version | grep BMI
     bool UseBMI1Instructions                      = true                                 {ARCH product} {default}
     bool UseBMI2Instructions                      = false                                {ARCH product} {default}

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Changes requested by rcastanedalo (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25415#pullrequestreview-2870679818