VCVTBIASPS2HF8S

VCVTBIASPS2HF8S is VCVTBIASPS2HF8 with saturation. Each single, from the third operand, gets the low 20 bits of the matching bias doubleword of the second operand added below E4M3’s last place and is then cut to E4M3’s 3 fraction bits, so that a bias of 0 truncates and random biases round stochastically; the bytes go to the low end of an xmm register under a write mask of one bit per byte. A sum of 512 or more, and an infinite single, give the largest value, 448 (7Eh or FEh), with the single’s sign; a NaN gives the NaN. The encoding is opcode 3Ah with no prefix in EVEX map 5, W0, in AVX10_V2_AUX of the ACE specification.

Two gaps in the specification#

ACE’s description and its pseudocode helper disagree in two places, and the Operation section gives UNDEFINED in both rather than choose. A sum from 480 to just under 512 falls on 7Fh: the table of saturation results gives 448, but the helper, which saturates only from 512, returns the NaN. And for results below 2^-6, the description keeps E4M3 denormals while the helper returns zeros, as on the page of the form without saturation. Between 2^-6 and 480 the result is the truncated sum, the same as VCVTBIASPS2HF8’s. MXCSR is neither read nor written, a denormal single counts as a zero, and nothing is flagged. VCVTPS2HF8S rounds to nearest even and has no such gap.

In our builds#

GCC 14 and Clang 23 have no option for AVX10_V2_AUX: Clang 23’s assembler and GNU as 2.44 reject the mnemonic, and ACE’s intrinsic _mm512_cvtbiaspss_hf8 is missing from Clang 23’s headers. Zen 5, whose CPUID reports no AVX10, raised #UD in all seven recorded cases.

Forms

Data flow

Where the elements of the sources go, element 0 on the right; results that depend on a control value are drawn for an example. How to read.

Operation

What the forms do, in pseudocode that our tests run against results recorded on real processors. How to read.

if not cpuid_leaf(7, 1)[3] >> 19 & 1:
    fault(UD)                    # CPUID: no AVX10
if not cpuid_leaf(0x24, 1)[2] >> 3 & 1:
    fault(UD)                    # no AVX10_V2_AUX
for i in range(VL // 32):
    if k1[i]:
        x = src3.f32[i]          # single i
        c = zmm2.d[i] & 0xFFFFF  # its bias: doubleword i's
                                 # bits 19:0
        s = x >> 24 & 0x80       # the sign, in place
        a = x & 0x7FFFFFFF       # |x|, as bits
        t = a + c                # the bias below E4M3's last
                                 # place at a's exponent
        e = t >> 23              # the sum's exponent
        if a > 0x7F800000:       # a NaN: the NaN
            r = s | 0x7F
        elif a == 0x7F800000:    # infinity: the largest, 448
            r = s | 0x7E
        elif a < 0x800000:       # zero or denormal: a zero
            r = s
        elif e > 120:            # 2^-6 and up: a normal
            r = (t >> 20) - 0x3C0
                                 # cut, the exponent rebiased
                                 # (127 to 7)
            if r == 0x7F:        # 480 to 512: ACE's helper
                                 # (16.1) gives the NaN, its
                                 # table (9.2.1) 448
                r = UNDEFINED
            else:
                r = s | min(r, 0x7E)  # 512 and up: 448
        else:                    # below 2^-6: ACE's helper
                                 # (16.1) gives a zero, its text
                                 # (9.2.1 and its table) keeps
                                 # denormal results
            r = UNDEFINED
        xmm1.hf8[i] = r
    elif zeroing:
        xmm1.hf8[i] = 0
    # else: xmm1.hf8[i] keeps its value
for i in range(VL // 32, 16):
    xmm1.b[i] = 0
# m32bcst: src3.f32[i] is the m32 for every i
# no flags; MXCSR unused, a denormal single a zero
# document: ACE 1.16.2
# xmm1 is zeroed from bit 128 up

Operands

Flags

None: XED records no flag that the forms test or change.

Exceptions

The exception class is XED's name for the set of exception conditions that a form follows. How to read.

All forms

AVX512-E4

Attributes

What else XED records about the forms. How to read.

All forms

write masking

Memory forms

embedded broadcast (tuple Full); memory fault suppression

CPUID and processors

Each form belongs to an XED ISA set. A form can be used when the processor reports every CPUID bit of one of its ISA set's alternatives.

More AVX10_V2_AUX instructions

• VCVTBF42HF8
• VCVTBF62HF8
• VCVTBF82BF4S
• VCVTBF82BF6S
• VCVTBF82PS
• VCVTBIASPS2BF8
• VCVTBIASPS2BF8S
• VCVTBIASPS2HF8
• VCVTHF62HF8
• VCVTHF82BF4S
• VCVTHF82HF6S
• VCVTHF82PS
• VCVTPS2BF8
• VCVTPS2BF8S
• VCVTPS2HF8
• VCVTPS2HF8S
• VCVTROPS2HF8
• VCVTROPS2HF8S
• VPMOVSSDB
• VUNPACKB