target/arm: Add some comments in Thumb decode

Add some comments to the Thumb decoder indicating what bits
of the instruction have been decoded at various points in
the code.

This is not an exhaustive set of comments; we're gradually
adding comments as we work with particular bits of the code.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-id: 20181002163556[email protected]

diff --git a/target/arm/translate.c b/target/arm/translate.c
index 25a8fe672f56a92d49ef7e4f51a09c3a47f83c5c..fcb33b8a50358ad8d9cb7985ef740324a953a2e6 100644 (file)
--- a/target/arm/translate.c
+++ b/target/arm/translate.c
@@ -10623,6 +10623,10 @@ static void disas_thumb2_insn(DisasContext *s, uint32_t insn)
             tmp2 = load_reg(s, rm);
             if ((insn & 0x70) != 0)
                 goto illegal_op;
+            /*
+             * 0b1111_1010_0xxx_xxxx_1111_xxxx_0000_xxxx:
+             *  - MOV, MOVS (register-shifted register), flagsetting
+             */
             op = (insn >> 21) & 3;
             logic_cc = (insn & (1 << 20)) != 0;
             gen_arm_shift_reg(tmp, op, tmp2, logic_cc);
@@ -11674,7 +11678,11 @@ static void disas_thumb_insn(DisasContext *s, uint32_t insn)
         rd = insn & 7;
         op = (insn >> 11) & 3;
         if (op == 3) {
-            /* add/subtract */
+            /*
+             * 0b0001_1xxx_xxxx_xxxx
+             *  - Add, subtract (three low registers)
+             *  - Add, subtract (two low registers and immediate)
+             */
             rn = (insn >> 3) & 7;
             tmp = load_reg(s, rn);
             if (insn & (1 << 10)) {
@@ -11711,7 +11719,10 @@ static void disas_thumb_insn(DisasContext *s, uint32_t insn)
         }
         break;
     case 2: case 3:
-        /* arithmetic large immediate */
+        /*
+         * 0b001x_xxxx_xxxx_xxxx
+         *  - Add, subtract, compare, move (one low register and immediate)
+         */
         op = (insn >> 11) & 3;
         rd = (insn >> 8) & 0x7;
         if (op == 0) { /* mov */
@@ -11848,7 +11859,10 @@ static void disas_thumb_insn(DisasContext *s, uint32_t insn)
             break;
         }
 
-        /* data processing register */
+        /*
+         * 0b0100_00xx_xxxx_xxxx
+         *  - Data-processing (two low registers)
+         */
         rd = insn & 7;
         rm = (insn >> 3) & 7;
         op = (insn >> 6) & 0xf;