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Comment by dmitrygr

1 day ago

> many, if not the vast majority, of aarch64 TBZ/TBNZ are probably branching on a boolean

None are. There is CBZ/CBNZ for that. https://www.scs.stanford.edu/~zyedidia/arm64/cbnz.html

It is just THAT useful to branch in a bit.

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dmitrygr

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dzaima  21 hours ago

Both clang and gcc do actually generate TBZ/TBNZ for checking a bool: https://godbolt.org/z/K6evhaxGT

  • dzaima  17 hours ago

    Some stats on an aarch64 binary of my current main project (1.6MB .text, 6600 symbols as per whatever "nm the-binary | wc -l" includes, from "objdump -d the-binary"):

        19546 /tbn?z/
    18029 /tbn?z.*, #0x0/  (but this includes boolean checks)
      224 /tbn?z.*, #0x1f/ (i.e. 32-bit x<0)
     1139 /tbn?z.*, #0x3f/ (i.e. 64-bit x<0)
      154 other immediates

    Said project doesn't do fixed bitfields much (there are some, but a chunk of those test multiple bits) so unsurprisingly not much. (I could imagine that the kernel has significantly more, but it's an edge-case (though perhaps an important one) of being basically massive amounts of fixed configurable glue)

    • camel-cdr  16 hours ago

      Did a quick grep over object files of a half build defconfig kernel:

          total: 15510
    #0:  3720
    #31: 2247
    #1:  1349
    #21: 1208
    #2:  810
    #3:  524
    #8:  493
    ...

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