8388926: RISC-V: Skip redundant zext.w in set_narrow_klass when bit 31 is clear

[zifeihan]

Hi, please consider.
MacroAssembler::set_narrow_klass uses li32 (lui + addiw) to materialize a narrow klass constant into a register. Since addiw sign-extends the 32-bit result to 64 bits, the function previously emitted a zext.w unconditionally to clear the upper 32 bits. However, sign extension only corrupts the upper bits when bit 31 of the narrow klass value is set (i.e., (int32_t)nk < 0). When bit 31 is clear, addiw naturally produces a correctly zero-extended result, making zext.w redundant.

Correctness Testing

• Run tier1-tier3 test with release on SG2044

Assembly log verification

The command to print the Assembly log is as follows:
./java -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly -XX:CompileCommand=compileonly,java.util.HashMap::putVal -version > assembly.log
We can observe that after the optimization, the instruction .insn 4, 0x080888bb has been successfully eliminated, this is a Zba extension instruction. After decoding, it is add.uw a7, a7, zero, which is equivalent to zext.w a7, a7

without this patch:

Compiled method (c2) 775   22             java.util.HashMap::putVal (300 bytes)
...
  0x0000003f8d09409e:   addiw	s2,zero,981
  0x0000003f8d0940a2:   slli	s2,s2,0x2a
  0x0000003f8d0940a4:   addi	s2,s2,1
  0x0000003f8d0940a6:   lui	a7,0x0                      ;   {metadata('java/util/LinkedHashMap')}
  0x0000003f8d0940aa:   addiw	a7,a7,2034 # 0x00000000000007f2
  0x0000003f8d0940ae:   .insn	4, 0x080888bb
  0x0000003f8d0940b2:   slli	a4,t2,0x3                   ;*aaload {reexecute=0 rethrow=0 return_oop=0}
                                                            ; - java.util.HashMap::putVal@40 (line 662)
  0x0000003f8d0940b6:   lui	a5,0x0                      ;   {metadata('java/util/HashMap')}
  0x0000003f8d0940ba:   addiw	a5,a5,973 # 0x00000000000003cd
  0x0000003f8d0940be:   .insn	4, 0x080787bb
  0x0000003f8d0940c2:   ld	t3,8(sp)
  0x0000003f8d0940c4:   addiw	s3,zero,985
  0x0000003f8d0940c8:   slli	s3,s3,0x2a
  0x0000003f8d0940ca:   addi	s3,s3,1
  0x0000003f8d0940cc:   ld	a2,16(sp)
  0x0000003f8d0940ce:   li	t4,1
  0x0000003f8d0940d0:   srli	a1,t3,0x3                   ;*putfield key {reexecute=0 rethrow=0 return_oop=0}

with this patch:

Compiled method (c2) 775   22             java.util.HashMap::putVal (300 bytes)
...
  0x0000003fa1093e1e:   addiw	s2,zero,981
  0x0000003fa1093e22:   slli	s2,s2,0x2a
  0x0000003fa1093e24:   addi	s2,s2,1
  0x0000003fa1093e26:   lui	a7,0x0                      ;   {metadata('java/util/LinkedHashMap')}
  0x0000003fa1093e2a:   addiw	a7,a7,2034 # 0x00000000000007f2
  0x0000003fa1093e2e:   slli	a4,t2,0x3                   ;*aaload {reexecute=0 rethrow=0 return_oop=0}
                                                            ; - java.util.HashMap::putVal@40 (line 662)
  0x0000003fa1093e32:   lui	a5,0x0                      ;   {metadata('java/util/HashMap')}
  0x0000003fa1093e36:   addiw	a5,a5,973 # 0x00000000000003cd
  0x0000003fa1093e3a:   ld	t3,8(sp)
  0x0000003fa1093e3c:   addiw	s3,zero,985
  0x0000003fa1093e40:   slli	s3,s3,0x2a
  0x0000003fa1093e42:   addi	s3,s3,1
  0x0000003fa1093e44:   ld	a2,16(sp)
  0x0000003fa1093e46:   li	t4,1
  0x0000003fa1093e48:   srli	a1,t3,0x3                   ;*putfield key {reexecute=0 rethrow=0 return_oop=0}

Performance Test:

specjbb2015 without this patch:

RUN RESULT: hbIR (max attempted) = 4579, hbIR (settled) = 4365, max-jOPS = 4350, critical-jOPS = 1129

specjbb2015 witch this patch:

RUN RESULT: hbIR (max attempted) = 4187, hbIR (settled) = 4038, max-jOPS = 4396, critical-jOPS = 1170

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Issue

• JDK-8388926: RISC-V: Skip redundant zext.w in set_narrow_klass when bit 31 is clear (Enhancement - P4)

Contributors

• gns <wangbingzhen.riscv@isrc.iscas.ac.cn>

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[zifeihan]

/contributor add gns wangbingzhen.riscv@isrc.iscas.ac.cn

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