liblzma: Add comments.

src/liblzma/lzma/lzma_decoder.c

@@ -474,6 +474,15 @@ lzma_decode(void *coder_ptr, lzma_dict *restrict dictptr,
 					// Variable number (1-5) of bits
 					// from a reverse bittree. This
 					// isn't worth manual unrolling.
+					//
+					// NOTE: Making one or many of the
+					// variables (probs, symbol, offset,
+					// or limit) local here (instead of
+					// using those declared outside the
+					// main loop) can affect code size
+					// and performance which isn't a
+					// surprise but it's not so clear
+					// what is the best.
 					do {
 						rc_bit_add_if_1(probs,
 								rep0, offset);

src/liblzma/rangecoder/range_decoder.h

@@ -305,8 +305,15 @@ do { \
 } while (0)
 
 
-// Decode one bit from variable-sized reverse bittree.
+// Decode one bit from variable-sized reverse bittree. The loop is done
-// The loop is done in the code that uses this macro.
+// in the code that uses this macro. This could be changed if the assembly
+// version benefited from having the loop done in assembly but it didn't
+// seem so in early 2024.
+//
+// Also, if the loop was done here, the loop counter would likely be local
+// to the macro so that it wouldn't modify yet another input variable.
+// If a _safe version of a macro with a loop was done then a modifiable
+// input variable couldn't be avoided though.
 #define rc_bit_add_if_1(probs, dest, value_to_add_if_1) \
 	rc_bit(probs[symbol], \
 		, \