Clever Geek Handbook

AVX-512 command encoding - {er} Meaning

There are several AVX-512 instructions in the Intel x86 instruction set reference that have an optional {er} in the instruction. For example, one form VADDPD is defined as

EVEX.NDS.512.66.0F.W1 58 /r
VADDPD zmm1 {k1}{z}, zmm2,
zmm3/m512/m64bcst{er}

What does {er} mean?

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From Intel SDM Volume 2A, 3.1.1.3 "Command Column in Operation Code Summary Table":

{er} - Indicates support for built-in rounding control, which applies only to the register-register form of the instruction. This also includes support for SAE (Suppress All Exceptions).



Section 2.6.8 bits earlier stated that {er}, if applicable, can be encoded into EVEX.L'L:

The static rounding control implemented in the EVEX coding system only applies to register-flavored floating point with rounding semantics on two different vector lengths: (i) scalar, (ii) 512 bits. In both cases, the EVEX.LL field expresses control over the rounding mode, overriding MXCSR.RC if EVEX.b is set. When EVEX.b is set, suppress all exceptions. The processor behaves as if all of the MXCSR masking controls are installed.

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