comp16-v1-skel: first cut at the logic for attempt 1 encoding, untested

[libreriscv.git] / lxo / ChangeLog

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 2020-11-28
 
+       * 532: Figured out and implemented the logic to infer mode
+       switching for best compression under attempt 1 proposed encoding,
+       namely with 10-bit insns, 16-bit insns, 16+16-bit insns, and
+       32-bit insns.  10-bit insns appear in uncompressed mode, and can
+       be followed by insns in either mode; 16-bit ones appear in
+       compressed mode, and can remain in compressed mode, or switch to
+       uncomprssed mode for 1 insn or for good; 16+16-bit ones appear in
+       compressed mode, and cannot switch modes; 32-bit ones appear only
+       in uncompressed mode, or in the single-insn slot after a 16-bit
+       that requests it.  If we find a 16-bit insn while we're in
+       uncompressed mode, use a 10-bit nop to tentatively switch.  Insns
+       that can be encoded in 10-bits, but appear in compressed mode, had
+       better be encoded in 16-bits, for that offers further subsequent
+       encoding options, without downsides for size estimation.  Insns
+       that can be encoded as 16+16-bit decay to 32-bit if in
+       uncompressed mode, or if, after a sequence thereof, a later insn
+       forces a switch to 32-bit mode without an intervening switching
+       insn.  Still missing: the code to select what insns can be encoded
+       in what modes.  (6:42)
+
        * 532: Implemented a skeleton for compression ratio estimation,
        initially with the simpler mode switching of the 8-bit nop,
        odd-address 16-bit insns.  Next, rewrite it for all the complexity