Integrated: 8297715: RISC-V: C2: Use single-bit instructions from the Zbs extension

[Fei Yang]

On Tue, 29 Nov 2022 03:35:12 GMT, Fei Yang <fyang at openjdk.org> wrote:

> The single-bit instructions from the Zbs extension provide a mechanism to set, clear,
> invert, or extract a single bit in a register. The bit is specified by its index.
> 
> Especially, the single-bit extract (immediate) instruction 'bexti rd, rs1, shamt' [1] performs:
> 
>   let index = shamt & (XLEN - 1);
>   X(rd) = (X(rs1) >> index) & 1;
> 
> 
> This instruction is a perfect match for following C2 sub-graph when integer immediate 'mask' is power of 2:
> 
>   Set dst (Conv2B (AndI src mask))
> 
> 
> The effect is that we could then optimize C2 JIT code for methods like [2]:
> Before:
> 
>   lhu  R28, [R11, #12]    # short, #@loadUS ! Field: com/sun/org/apache/xerces/internal/dom/NodeImpl.flags
>   andi  R7, R28, #8       #@andI_reg_imm
>   snez  R10, R7           #@convI2Bool
> 
> 
> After:
> 
>   lhu  R28, [R11, #12]    # short, #@loadUS ! Field: com/sun/org/apache/xerces/internal/dom/NodeImpl.flags
>   bexti  R10, R28, 3      #
> 
> Note that I am not adding a matching rule for long->bool case as I see C2 compiler currently only catches and builds int->bool conversions [3]. I guess there might be no such use cases for long->bool conversion in the real-world.
> 
> Testing: Tier1-3 hotspot & jdk tested with QEMU (JTREG="VM_OPTIONS=-XX:+UnlockExperimentalVMOptions -XX:+UseZbs").
> 
> [1] https://github.com/riscv/riscv-bitmanip/blob/main/bitmanip/insns/bexti.adoc
> 
> [2] https://github.com/openjdk/jdk/blob/master/src/java.xml/share/classes/com/sun/org/apache/xerces/internal/dom/NodeImpl.java#L1936
> 
> [3] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/cfgnode.cpp#L1435

This pull request has now been integrated.

Changeset: d50015af
Author:    Fei Yang <fyang at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/d50015af99f44909bf71fd2de97546d47cda86d6
Stats:     26 lines in 4 files changed: 24 ins; 0 del; 2 mod

8297715: RISC-V: C2: Use single-bit instructions from the Zbs extension

Reviewed-by: fjiang, yadongwang, shade

-------------

PR: https://git.openjdk.org/jdk/pull/11406