[FMV][AArch64] Remove features which expose non exploitable runtime b…

…ehavior. (#355)

Feature `rpres` allows an increase in the precision of the
single-precision
floating-point reciprocal estimate and reciprocal square root estimate
from an 8-bit mantissa to a 12-bit mantissa depending on the value
FPCR.AH
of the Floating-point Control Register.

Similarly, `ebf16` allows existing floating-point instructions (BFDOT,
BFMMLA,
BFMOPA, BFMOPS, and BFVDOT) to change numeric behaviour depending on the
value FPCR.EBF of the Floating-point Control Register.

Feature `memtag3` allows Tag Check Faults to change behaviour depending
on
the value SCTLR_ELx.{TCF, TCF0} of the System Control Register.

The runtime detection in FMV does not examine the content of control
registers, therefore runtime dispatch cannot be based on that. One may
argue there is value in altering the control register from a version,
for example "msr dit, \#1" if the feature is available on hardware. The
registers FPCR and SCTLR_ELx can be accessed in the absence of rpres,
ebf16, and memtag3, making it hard to justify adding them to the
compiler.

main/acle.md

@@ -419,6 +419,7 @@ Armv8.4-A [[ARMARMv84]](#ARMARMv84). Support is added for the Dot Product intrin
 * Unified Function Multi Versioning features memtag and memtag2.
 * Unified Function Multi Versioning features aes and pmull.
 * Unified Function Multi Versioning features sve2-aes and sve2-pmull128.
+* Removed Function Multi Versioning features ebf16, memtag3, and rpres.
 * Fixed range of operand `o0` (too small) in AArch64 system register designations.
 * Fixed SVE2.1 quadword gather load/scatter store intrinsics.
 
@@ -2803,8 +2804,6 @@ The following table lists the architectures feature mapping for AArch64
    | 260           | `FEAT_DGH`               | dgh           | ```ID_AA64ISAR1_EL1.DGH >= 0b0001```      |
    | 270           | `FEAT_I8MM`              | i8mm          | ```ID_AA64ISAR1_EL1.I8MM >= 0b0001```     |
    | 280           | `FEAT_BF16`              | bf16          | ```ID_AA64ISAR1_EL1.BF16 >= 0b0001```     |
-   | 290           | `FEAT_EBF16`             | ebf16         | ```ID_AA64ISAR1_EL1.BF16 >= 0b0010```     |
-   | 300           | `FEAT_RPRES`             | rpres         | ```ID_AA64ISAR2_EL1.RPRES >= 0b0001```    |
    | 310           | `FEAT_SVE`               | sve           | ```ID_AA64PFR0_EL1.SVE >= 0b0001```       |
    | 320           | `FEAT_BF16`              | sve-bf16      | ```ID_AA64ZFR0_EL1.BF16 >= 0b0001```      |
    | 330           | `FEAT_EBF16`             | sve-ebf16     | ```ID_AA64ZFR0_EL1.BF16 >= 0b0010```      |
@@ -2818,7 +2817,6 @@ The following table lists the architectures feature mapping for AArch64
    | 420           | `FEAT_SM3`, `FEAT_SVE_SM4` | sve2-sm4    | ```ID_AA64ZFR0_EL1.SM4 >= 0b0001```       |
    | 430           | `FEAT_SME`               | sme           | ```ID_AA64PFR1_EL1.SME >= 0b0001```       |
    | 440           | `FEAT_MTE`, `FEAT_MTE2`  | memtag        | ```ID_AA64PFR1_EL1.MTE >= 0b0010```       |
-   | 460           | `FEAT_MTE3`              | memtag3       | ```ID_AA64PFR1_EL1.MTE >= 0b0011```       |
    | 470           | `FEAT_SB`                | sb            | ```ID_AA64ISAR1_EL1.SB >= 0b0001```       |
    | 480           | `FEAT_SPECRES`           | predres       | ```ID_AA64ISAR1_EL1.SPECRES >= 0b0001```  |
    | 490           | `FEAT_SSBS`, `FEAT_SSBS2`| ssbs          | ```ID_AA64PFR1_EL1.SSBS >= 0b0010```      |