Hacker News Reader

Comment by PeCaN | Hacker News Reader

It's not like someone is proposing some crazy new instruction to do vector math on binary coded decimals while also calculating CRC32 values as a byproduct. It's conditional move. Every ISA I can think of has that.

View on HN

This prompted me to look through some RISC ISAs (+x86), there may be errors since I made just a cursory pass.

Seems the following have conditional moves: MIPS since IV, Alpha, x86 since PPRo, SPARC since SPARCv9

The following seem to omit conditional moves: AVR, PowerPC, Hitachi SH, MIPS I-III, x86 up to Pentium, SPARC up to SPARCv8, ARM, PA-RISC (?)

PA-RISC, PowerPC, ARM at least do a lot of predication and make a high investment to conditional operations (by way of dedicating a lot of bits in insn layout to it), but also end up using it a lot more often than conditional move tends to be used.

ARMv7's Thumb2 has general predication of hammocks via "if-then", and ARM itself had general predication. ARMv8 has conditional select, which is quite a bit richer than conditional move. POWER has "isel". Seeing an ISA evolve a conditional move later in life is pretty strong evidence that it was useful enough to include. So would modify your list to be:

ISAs that evolved conditional move:

  - MIPS
  - SPARC
  - x86
  - POWER (isel)

ISAs that started life with it:

  - ARM (via general predication)
  - Alpha
  - IA64 (via general predication)

To be clear for anyone not so up-to-speed on this: what AArch64 has (conditional select) is strictly less expressive than AArch32 (general predication).

The take away there is that general predication was found to be overly complex where the vast (vast!) majority of the benefit can be modelled with conditional select.

Its less than general predication, but a little bit more than cmov/csel. The second argument can be optionally incremented and/or complemented. Combined with the dedicated zero register, you can do all sorts of interesting things to turn condition-generating instructions into data. A few interesting ones include:

   y = cond ? 0 : -1;
   y = cond ? x : -x;
   x = cond ? 0 : x+1;  //< look ma, circular addressing!

Good list.

Observation re list of ISAs that evolved conditional move vs ISAs that omit conditional move: MIPS, POWER, x86, SPARC all targeted high power "fat core" applications at the point where it got added. AVR, Hitachi SH, PowerPC didn't add it while being driven more by low power / embedded applications. And many ISAs continued to see wide use in the pre-cmov versions of the ISA in embedded space (eg MIPS) after the additions. (PowerPC even removed it when being modeled after POWER)

Reply on news.ycombinator.com