[15] RFR: 8248048: ZGC: AArch64: SIGILL in load barrier register spilling

Stefan Karlsson stefan.karlsson at oracle.com
Fri Jun 26 08:50:53 UTC 2020


Hi all,

Please review this patch to fix a ZGC load barrier register spilling bug.

https://cr.openjdk.java.net/~stefank/8248048/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8248048

The JVM crashed with an ILL_ILLOPC when executing this instruction in 
our load barrier stub:

   ldp q31, q31, [sp, #224]

The entire load barrier stub:

    0x0000ffff998ab964:    stp    x10, x13, [sp, #-32]!
    0x0000ffff998ab968:    stp    x14, x17, [sp, #16]
    0x0000ffff998ab96c:    stp    q1, q2, [sp, #-256]!
    0x0000ffff998ab970:    stp    q3, q19, [sp, #32]
    0x0000ffff998ab974:    stp    q20, q21, [sp, #64]
    0x0000ffff998ab978:    stp    q22, q23, [sp, #96]
    0x0000ffff998ab97c:    stp    q24, q25, [sp, #128]
    0x0000ffff998ab980:    stp    q26, q28, [sp, #160]
    0x0000ffff998ab984:    stp    q29, q30, [sp, #192]
    0x0000ffff998ab988:    stp    q31, q31, [sp, #224]
    0x0000ffff998ab98c:    sub    x1, x10, #0x0
    0x0000ffff998ab990:    mov    x0, x11
    0x0000ffff998ab994:    mov    x8, #0xfc28                    // #64552
    0x0000ffff998ab998:    movk    x8, #0xaf11, lsl #16
    0x0000ffff998ab99c:    movk    x8, #0xffff, lsl #32
    0x0000ffff998ab9a0:    blr    x8 ; branch into the JVM
    0x0000ffff998ab9a4:    mov    x11, x0
    0x0000ffff998ab9a8:    ldp    q3, q19, [sp, #32]
    0x0000ffff998ab9ac:    ldp    q20, q21, [sp, #64]
    0x0000ffff998ab9b0:    ldp    q22, q23, [sp, #96]
    0x0000ffff998ab9b4:    ldp    q24, q25, [sp, #128]
    0x0000ffff998ab9b8:    ldp    q26, q28, [sp, #160]
    0x0000ffff998ab9bc:    ldp    q29, q30, [sp, #192]
=> 0x0000ffff998ab9c0:    ldp    q31, q31, [sp, #224]
    0x0000ffff998ab9c4:    ldp    q1, q2, [sp], #256
    0x0000ffff998ab9c8:    ldp    x14, x17, [sp, #16]
    0x0000ffff998ab9cc:    ldp    x10, x13, [sp], #32
    0x0000ffff998ab9d0:    b    0xffff998aa718

It seems to be illegal to use the same register twice when loading into 
a pair of registers. I verified that that was the problem, and not the 
usage of zr (see below) that caused some weird encoding, by changing the 
code to always generate stp/ldp with the same register:

=> 0x0000ffff757d22fc:    ldp    q20, q20, [sp, #32] ; Crash here as well
    0x0000ffff757d2300:    ldp    q21, q21, [sp, #48]
    0x0000ffff757d2304:    ldp    q22, q22, [sp, #64]

The code that generates this instruction is MacroAssembler::push_fp, 
which spills the necessary registers in pairs with stp/ldp calls. If the 
number of registers to spill is odd it needs to deal with one of the 
registers separately. This is done by adding a dummy register here:

2136 regs[count++] = zr->encoding_nocheck();
2137 count &= ~1; // Only push an even number of regs

This scheme seems to work for the normal registers 
(MacroAssembler::push), but the usage of zr seems dubious when we're 
dealing with the fp/simd version of stp/ldp.

My proposed patch replaces the stp/ldp for the odd numbered register 
with the single-register versions: str/ldr. I make sure to keep the 
stack 16 bytes aligned by still bumping 16 bytes, but skipping the 
store/load to the second 8 bytes half.

Note that right now MacroAssembler::push_fp is only used by ZGC.

This fixes the crash. I've run this code through jtreg groups :tier1, 
tier2, tier3, and an Oracle-internal stress suite without any new problems.

The smallest reproducer I have is:
make -C ../build/fastdebug test TEST=test/jdk/java/util/concurrent/ 
JTREG="JAVA_OPTIONS=-XX:+UseZGC"

Does this look OK?

Thanks,
StefanK


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