From: Peter Maydell <peter.maydell@linaro.org>
Date: Mon, 8 Oct 2018 13:55:04 +0000 (+0100)
Subject: target/arm: Add some comments in Thumb decode
X-Git-Tag: qemu-xen-4.13.0-rc1~631^2~9
X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=a2d12f0f34e9c5ef8a193556fde983aa186fa73a;p=qemu-xen.git

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 <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20181002163556.10279-6-peter.maydell@linaro.org
---

diff --git a/target/arm/translate.c b/target/arm/translate.c
index 25a8fe672f..fcb33b8a50 100644
--- 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;