No, unless I'm missing something clever, ptest
with two unknown registers is generally not useful for checking some property about both of them. (Other than obvious stuff you'd already want a bitwise-AND for, like intersection between two bitmaps).
To test two registers for both being all-zero, OR them together and PTEST that against itself.
ptest xmm0, xmm1
produces two results:
- ZF = is
xmm0 & xmm1
all-zero?
- CF = is
(~xmm0) & xmm1
all-zero?
If the second vector is all-zero, the flags don't depend at all on the bits in the first vector.
It may be useful to think of the "is-all-zero" checks as a NOT(bitwise horizontal-OR())
of the AND and ANDNOT results. But probably not, because that's too many steps for my brain to think through easily. That sequence of vertical-AND and then horizontal-OR does maybe make it easier to understand why PTEST doesn't tell you much about a combination of two unknown registers, just like the integer TEST instruction.
Here's a truth table for a 2-bit ptest a,mask
. Hopefully this helps in thinking about mixes of zeros and ones with 128b inputs.
Note that CF(a,mask) == ZF(~a,mask)
.
a mask ZF CF
00 00 1 1
01 00 1 1
10 00 1 1
11 00 1 1
00 01 1 0
01 01 0 1
10 01 1 0
11 01 0 1
00 10 1 0
01 10 1 0
10 10 0 1
11 10 0 1
00 11 1 0
01 11 0 0
10 11 0 0
11 11 0 1
Intel's intrinsics guide lists 2 interesting intrinsics for it. Note the naming of the args: a
and mask
are a clue that they tell you about the parts of a
selected by a known AND-mask.
_mm_test_mix_ones_zeros (__m128i a, __m128i mask)
: returns (ZF == 0 && CF == 0)
_mm_test_all_zeros (__m128i a, __m128i mask)
: returns ZF
There's also the more simply-named versions:
int _mm_testc_si128 (__m128i a, __m128i b)
: returns CF
int _mm_testnzc_si128 (__m128i a, __m128i b)
: returns (ZF == 0 && CF == 0)
int _mm_testz_si128 (__m128i a, __m128i b)
: returns ZF
There are AVX2 __m256i
versions of those intrinsics, but the guide only lists the all_zeros and mix_ones_zeros alternate-name versions for __m128i
operands.
If you want to test some other condition from C or C++, you should use testc
and testz
with the same operands, and hope that your compiler realizes that it only needs to do one PTEST, and hopefully even use a single JCC, SETCC, or CMOVCC to implement your logic. (I'd recommend checking the asm, at least for the compiler you care about most.)
Note that _mm_testz_si128(v, set1(0xff))
is always the same as _mm_testz_si128(v,v)
, because that's how AND works. But that's not true for the CF result.
You can check for a vector being all-ones using
bool is_all_ones = _mm_testc_si128(v, _mm_set1_epi8(0xff));
This is probably no faster, but smaller code-size, than a PCMPEQB against a vector of all-ones, then the usual movemask + cmp. It doesn't avoid the need for a vector constant.
PTEST does have the advantage that it doesn't destroy either input operand, even without AVX.