8320379: C2: Sort spilling/unspilling sequence for better ld/st merging into ldp/stp on AArch64 #16754

fg1417 · 2023-11-21T07:15:15Z

Macro-assembler on aarch64 can merge adjacent loads or stores into ldp/stp.[1]

For example, it can merge:

str     w20, [sp, #16]
str     w10, [sp, #20]

into

stp     w20, w10, [sp, #16]

But C2 may generate a sequence like:

str     x21, [sp, #8]
str     w20, [sp, #16]
str     x19, [sp, #24] <---
str     w10, [sp, #20] <--- Before sorting
str     x11, [sp, #40]
str     w13, [sp, #48]
str     x16, [sp, #56]

We can't do any merging for non-adjacent loads or stores.

The patch is to sort the spilling or unspilling sequence in the order of offset during instruction scheduling and bundling phase. After that, we can get a new sequence:

str     x21, [sp, #8]
str     w20, [sp, #16]
str     w10, [sp, #20] <---
str     x19, [sp, #24] <--- After sorting
str     x11, [sp, #40]
str     w13, [sp, #48]
str     x16, [sp, #56]

Then macro-assembler can do ld/st merging:

str     x21, [sp, #8]
stp     w20, w10, [sp, #16] <--- Merged
str     x19, [sp, #24]
str     x11, [sp, #40]
str     w13, [sp, #48]
str     x16, [sp, #56]

To justify the patch, we run HelloWorld.java

public class HelloWorld {
    public static void main(String [] args) {
        System.out.println("Hello World!");
    }
}

with java -Xcomp -XX:-TieredCompilation HelloWorld.

Before the patch, macro-assembler can do ld/st merging for 3688 times. After the patch, the number of ld/st merging increases to 3871 times, by ~5 %.

Tested tier1~3 on x86 and AArch64.

Progress

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Issue

JDK-8320379: C2: Sort spilling/unspilling sequence for better ld/st merging into ldp/stp on AArch64 (Enhancement - P4)

Reviewers

Andrew Haley (@theRealAph - Reviewer)
Vladimir Kozlov (@vnkozlov - Reviewer)

Reviewing

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$ git pull https://git.openjdk.org/jdk.git pull/16754/head

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$ git pr checkout 16754

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Webrev

Link to Webrev Comment

…ng into ldp/stp on AArch64 Macro-assembler on aarch64 can merge adjacent loads or stores into ldp/stp[1]. For example, it can merge: ``` str w20, [sp, openjdk#16] str w10, [sp, openjdk#20] ``` into ``` stp w20, w10, [sp, openjdk#16] ``` But C2 may generate a sequence like: ``` str x21, [sp, openjdk#8] str w20, [sp, openjdk#16] str x19, [sp, openjdk#24] <--- str w10, [sp, openjdk#20] <--- Before sorting str x11, [sp, openjdk#40] str w13, [sp, openjdk#48] str x16, [sp, openjdk#56] ``` We can't do any merging for non-adjacent loads or stores. The patch is to sort the spilling or unspilling sequence in the order of offset during instruction scheduling and bundling phase. After that, we can get a new sequence: ``` str x21, [sp, openjdk#8] str w20, [sp, openjdk#16] str w10, [sp, openjdk#20] <--- str x19, [sp, openjdk#24] <--- After sorting str x11, [sp, openjdk#40] str w13, [sp, openjdk#48] str x16, [sp, openjdk#56] ``` Then macro-assembler can do ld/st merging: ``` str x21, [sp, openjdk#8] stp w20, w10, [sp, openjdk#16] <--- Merged str x19, [sp, openjdk#24] str x11, [sp, openjdk#40] str w13, [sp, openjdk#48] str x16, [sp, openjdk#56] ``` To justify the patch, we run `HelloWorld.java` ``` public class HelloWorld { public static void main(String [] args) { System.out.println("Hello World!"); } } ``` with `java -Xcomp -XX:-TieredCompilation HelloWorld`. Before the patch, macro-assembler can do ld/st merging for 3688 times. After the patch, the number of ld/st merging increases to 3871 times, by ~5 %. Tested tier1~3 on x86 and AArch64. [1] https://github.com/openjdk/jdk/blob/a95062b39a431b4937ab6e9e73de4d2b8ea1ac49/src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp#L2079

bridgekeeper · 2023-11-21T07:16:52Z

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openjdk · 2023-11-21T07:18:26Z

@fg1417 The following label will be automatically applied to this pull request:

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mlbridge · 2023-11-21T07:22:40Z