This flowchart hides the most awful parts (IMO) of x86 prefixes: some combinations of prefixes are invalid but still parsed and executed, like combining two segment overrides, or placing a legacy prefix after a REX prefix.
The CPU also doesn't care if you use prefixes that aren't valid for a specific instruction, for example a REP on a non-repeatable instruction. The LOCK prefix is the only prefix that makes the sane choice to reject invalid combinations, rather than silently accept them.
Also, the (E)VEX prefix doesn't behave like the other prefixes: it must be placed last, and can therefore only appear once. All other prefixes can be repeated.
> The CPU also doesn't care if you use prefixes that aren't valid for a specific instruction, for example a REP on a non-repeatable instruction.
This is one of the reasons why the x86 could be extended so much. PAUSE is just REP NOP, for example. Segment prefixes in front of conditional branches were used as static branch prediction hints (which I believe have returned in some newer Intel CPUs). Useful if you want to make a hint on newer CPUs that is harmless on older CPUs.
Some prefixes have become part of the encoding for certain SIMD instructions, but that is a different case because those prefixes aren't hints.
Fun little tidbit: The 0x40-0x4f range used for the REX prefix actually clashes with the single-byte encodings for increment/decrement.
When AMD designed the 64 bit extension, they had run out of available single-byte opcodes to use as a prefix and decided to re-use those. The INC/DEC instructions are still available in 64 bit mode, but not in their single-byte encodings.
where there are links to a couple of patents filed by Intel in 2000, about a 64-bit extension of the x86 ISA, which had been implemented in Pentium 4, but which had been nonetheless disabled and hidden from the users, in order to not compete with Itanium.
The page explains the content of the patents.
As already mentioned by another poster, at least on Firefox you have to open a tab and then copy this link there, to avoid being identified as an "undesirable" :-)
I've seen benchmarks that go both ways in terms of a "winner" but in terms of overall variance there seems to be very little. There are some cases where ARM64 or RISCV do better and there are some cases where x86_64 does better. I can't see code density being a relevant factor when picking one ISA over another.
We've got good compilers now anyways.. outside of power consumption.. the ISA wars are dead.
Technically, code density still matters - because both L1 cache memory and L1 instruction fetch misses are very expensive.
But as you point out, code density gets far less attention in tech circles these days. And higher-level decision makers rightfully focus on higher-level system performance metrics.
The CPU also doesn't care if you use prefixes that aren't valid for a specific instruction, for example a REP on a non-repeatable instruction. The LOCK prefix is the only prefix that makes the sane choice to reject invalid combinations, rather than silently accept them.
Also, the (E)VEX prefix doesn't behave like the other prefixes: it must be placed last, and can therefore only appear once. All other prefixes can be repeated.
This is one of the reasons why the x86 could be extended so much. PAUSE is just REP NOP, for example. Segment prefixes in front of conditional branches were used as static branch prediction hints (which I believe have returned in some newer Intel CPUs). Useful if you want to make a hint on newer CPUs that is harmless on older CPUs.
Some prefixes have become part of the encoding for certain SIMD instructions, but that is a different case because those prefixes aren't hints.
The following is pulled in from `https://soc.me/assets/js/turnBack.js`:
I wonder why Reddit is "temporarily not undesirable".When AMD designed the 64 bit extension, they had run out of available single-byte opcodes to use as a prefix and decided to re-use those. The INC/DEC instructions are still available in 64 bit mode, but not in their single-byte encodings.
https://soc.me/interfaces/intels-original-64bit-extensions-f...
where there are links to a couple of patents filed by Intel in 2000, about a 64-bit extension of the x86 ISA, which had been implemented in Pentium 4, but which had been nonetheless disabled and hidden from the users, in order to not compete with Itanium.
The page explains the content of the patents.
As already mentioned by another poster, at least on Firefox you have to open a tab and then copy this link there, to avoid being identified as an "undesirable" :-)
I've seen benchmarks that go both ways in terms of a "winner" but in terms of overall variance there seems to be very little. There are some cases where ARM64 or RISCV do better and there are some cases where x86_64 does better. I can't see code density being a relevant factor when picking one ISA over another.
We've got good compilers now anyways.. outside of power consumption.. the ISA wars are dead.
But as you point out, code density gets far less attention in tech circles these days. And higher-level decision makers rightfully focus on higher-level system performance metrics.