Add fast path when extraction leads to intnat #90
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caml_z.c
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| @@ -934,6 +934,30 @@ CAMLprim value ml_z_extract(value arg, value off, value len) | |||
| if (x >= 0) return Val_long(x); | |||
| /* If x < 0, fall through slow path */ | |||
| } | |||
| } else if (l <= Z_BASE2_LENGTH_OP) { | |||
| Z_ARG(arg); | |||
| c1 = o / Z_LIMB_BITS; | |||
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If anyone else reads this and wonders whether it wouldn't be better to convert o to an unsigned integer type before computing / and %, the answer is, “with GCC and Clang at -O, yes, although not at -O2. See https://gcc.godbolt.org/z/Ya4vxW ”
I would recommend casting o to uintnat before these operations.
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Note that I simply reused what I found below: this part is the same as the slow path. So if needed, it should be changed in both places.
Yet I would have directly used corresponding bitwise operation >> and & but I though that / and % were still clearer and that the code would be compiled with optimisation so we would have not to bother with this kind of subtlety.
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The new version should address this issue.
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@pascal-cuoq @antoinemine |
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@antoinemine @xavierleroy |
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Yes, it's nice to handle the "small" case in a separate, "noalloc" C function. However, I haven't reviewed the bit-twiddling that implements this "small" case. |
caml_z.c
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| if (csz > 0) { | ||
| if (c2) { | ||
| x = ptr_arg[c1] >> c2; | ||
| if ((o + l > (intnat)Z_LIMB_BITS) && (csz > 1)) |
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After careful reading, I think that the test should be ((c2 + l > (intnat)Z_LIMB_BITS) && (csz > 1)) (c2 instead of o).
Proposition for #88.
At least, it passes the
make tests.