[Qemu-devel] [PULL 08/13] target/arm: Pull Thumb insn word loads up to top level

[Peter Maydell <peter.maydell@linaro.org>]

Refactor the Thumb decode to do the loads of the instruction words at
the top level rather than only loading the second half of a 32-bit
Thumb insn in the middle of the decode.

This is simple apart from the awkward case of Thumb1, where the
BL/BLX prefix and suffix instructions live in what in Thumb2 is the
32-bit insn space.  To handle these we decode enough to identify
whether we're looking at a prefix/suffix that we handle as a 16 bit
insn, or a prefix that we're going to merge with the following suffix
to consider as a 32 bit insn.  The translation of the 16 bit cases
then moves from disas_thumb2_insn() to disas_thumb_insn().

The refactoring has the benefit that we don't need to pass the
CPUARMState* down into the decoder code any more, but the major
reason for doing this is that some Thumb instructions must be always
unconditional regardless of the IT state bits, so we need to know the
whole insn before we emit the "skip this insn if the IT bits and cond
state tell us to" code.  (The always unconditional insns are BKPT,
HLT and SG; the last of these is 32 bits.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1507556919-24992-7-git-send-email-peter.maydell@linaro.org
---
 target/arm/translate.c | 178 ++++++++++++++++++++++++++++++-------------------
 1 file changed, 108 insertions(+), 70 deletions(-)

diff --git a/target/arm/translate.c b/target/arm/translate.c
index 530a5c4..19c136c 100644
--- a/target/arm/translate.c
+++ b/target/arm/translate.c
@@ -9620,6 +9620,44 @@ static void disas_arm_insn(DisasContext *s, unsigned int insn)
     }
 }
 
+static bool thumb_insn_is_16bit(DisasContext *s, uint32_t insn)
+{
+    /* Return true if this is a 16 bit instruction. We must be precise
+     * about this (matching the decode).  We assume that s->pc still
+     * points to the first 16 bits of the insn.
+     */
+    if ((insn >> 11) < 0x1d) {
+        /* Definitely a 16-bit instruction */
+        return true;
+    }
+
+    /* Top five bits 0b11101 / 0b11110 / 0b11111 : this is the
+     * first half of a 32-bit Thumb insn. Thumb-1 cores might
+     * end up actually treating this as two 16-bit insns, though,
+     * if it's half of a bl/blx pair that might span a page boundary.
+     */
+    if (arm_dc_feature(s, ARM_FEATURE_THUMB2)) {
+        /* Thumb2 cores (including all M profile ones) always treat
+         * 32-bit insns as 32-bit.
+         */
+        return false;
+    }
+
+    if ((insn >> 11) == 0x1e && (s->pc < s->next_page_start - 3)) {
+        /* 0b1111_0xxx_xxxx_xxxx : BL/BLX prefix, and the suffix
+         * is not on the next page; we merge this into a 32-bit
+         * insn.
+         */
+        return false;
+    }
+    /* 0b1110_1xxx_xxxx_xxxx : BLX suffix (or UNDEF);
+     * 0b1111_1xxx_xxxx_xxxx : BL suffix;
+     * 0b1111_0xxx_xxxx_xxxx : BL/BLX prefix on the end of a page
+     *  -- handle as single 16 bit insn
+     */
+    return true;
+}
+
 /* Return true if this is a Thumb-2 logical op.  */
 static int
 thumb2_logic_op(int op)
@@ -9705,9 +9743,9 @@ gen_thumb2_data_op(DisasContext *s, int op, int conds, uint32_t shifter_out,
 
 /* Translate a 32-bit thumb instruction.  Returns nonzero if the instruction
    is not legal.  */
-static int disas_thumb2_insn(CPUARMState *env, DisasContext *s, uint16_t insn_hw1)
+static int disas_thumb2_insn(DisasContext *s, uint32_t insn)
 {
-    uint32_t insn, imm, shift, offset;
+    uint32_t imm, shift, offset;
     uint32_t rd, rn, rm, rs;
     TCGv_i32 tmp;
     TCGv_i32 tmp2;
@@ -9719,51 +9757,9 @@ static int disas_thumb2_insn(CPUARMState *env, DisasContext *s, uint16_t insn_hw
     int conds;
     int logic_cc;
 
-    if (!arm_dc_feature(s, ARM_FEATURE_THUMB2)) {
-        /* Thumb-1 cores may need to treat bl and blx as a pair of
-           16-bit instructions to get correct prefetch abort behavior.  */
-        insn = insn_hw1;
-        if ((insn & (1 << 12)) == 0) {
-            ARCH(5);
-            /* Second half of blx.  */
-            offset = ((insn & 0x7ff) << 1);
-            tmp = load_reg(s, 14);
-            tcg_gen_addi_i32(tmp, tmp, offset);
-            tcg_gen_andi_i32(tmp, tmp, 0xfffffffc);
-
-            tmp2 = tcg_temp_new_i32();
-            tcg_gen_movi_i32(tmp2, s->pc | 1);
-            store_reg(s, 14, tmp2);
-            gen_bx(s, tmp);
-            return 0;
-        }
-        if (insn & (1 << 11)) {
-            /* Second half of bl.  */
-            offset = ((insn & 0x7ff) << 1) | 1;
-            tmp = load_reg(s, 14);
-            tcg_gen_addi_i32(tmp, tmp, offset);
-
-            tmp2 = tcg_temp_new_i32();
-            tcg_gen_movi_i32(tmp2, s->pc | 1);
-            store_reg(s, 14, tmp2);
-            gen_bx(s, tmp);
-            return 0;
-        }
-        if ((s->pc & ~TARGET_PAGE_MASK) == 0) {
-            /* Instruction spans a page boundary.  Implement it as two
-               16-bit instructions in case the second half causes an
-               prefetch abort.  */
-            offset = ((int32_t)insn << 21) >> 9;
-            tcg_gen_movi_i32(cpu_R[14], s->pc + 2 + offset);
-            return 0;
-        }
-        /* Fall through to 32-bit decode.  */
-    }
-
-    insn = arm_lduw_code(env, s->pc, s->sctlr_b);
-    s->pc += 2;
-    insn |= (uint32_t)insn_hw1 << 16;
-
+    /* The only 32 bit insn that's allowed for Thumb1 is the combined
+     * BL/BLX prefix and suffix.
+     */
     if ((insn & 0xf800e800) != 0xf000e800) {
         ARCH(6T2);
     }
@@ -11078,27 +11074,15 @@ illegal_op:
     return 1;
 }
 
-static void disas_thumb_insn(CPUARMState *env, DisasContext *s)
+static void disas_thumb_insn(DisasContext *s, uint32_t insn)
 {
-    uint32_t val, insn, op, rm, rn, rd, shift, cond;
+    uint32_t val, op, rm, rn, rd, shift, cond;
     int32_t offset;
     int i;
     TCGv_i32 tmp;
     TCGv_i32 tmp2;
     TCGv_i32 addr;
 
-    if (s->condexec_mask) {
-        cond = s->condexec_cond;
-        if (cond != 0x0e) {     /* Skip conditional when condition is AL. */
-          s->condlabel = gen_new_label();
-          arm_gen_test_cc(cond ^ 1, s->condlabel);
-          s->condjmp = 1;
-        }
-    }
-
-    insn = arm_lduw_code(env, s->pc, s->sctlr_b);
-    s->pc += 2;
-
     switch (insn >> 12) {
     case 0: case 1:
 
@@ -11829,8 +11813,21 @@ static void disas_thumb_insn(CPUARMState *env, DisasContext *s)
 
     case 14:
         if (insn & (1 << 11)) {
-            if (disas_thumb2_insn(env, s, insn))
-              goto undef32;
+            /* thumb_insn_is_16bit() ensures we can't get here for
+             * a Thumb2 CPU, so this must be a thumb1 split BL/BLX:
+             * 0b1110_1xxx_xxxx_xxxx : BLX suffix (or UNDEF)
+             */
+            assert(!arm_dc_feature(s, ARM_FEATURE_THUMB2));
+            ARCH(5);
+            offset = ((insn & 0x7ff) << 1);
+            tmp = load_reg(s, 14);
+            tcg_gen_addi_i32(tmp, tmp, offset);
+            tcg_gen_andi_i32(tmp, tmp, 0xfffffffc);
+
+            tmp2 = tcg_temp_new_i32();
+            tcg_gen_movi_i32(tmp2, s->pc | 1);
+            store_reg(s, 14, tmp2);
+            gen_bx(s, tmp);
             break;
         }
         /* unconditional branch */
@@ -11841,15 +11838,30 @@ static void disas_thumb_insn(CPUARMState *env, DisasContext *s)
         break;
 
     case 15:
-        if (disas_thumb2_insn(env, s, insn))
-            goto undef32;
+        /* thumb_insn_is_16bit() ensures we can't get here for
+         * a Thumb2 CPU, so this must be a thumb1 split BL/BLX.
+         */
+        assert(!arm_dc_feature(s, ARM_FEATURE_THUMB2));
+
+        if (insn & (1 << 11)) {
+            /* 0b1111_1xxx_xxxx_xxxx : BL suffix */
+            offset = ((insn & 0x7ff) << 1) | 1;
+            tmp = load_reg(s, 14);
+            tcg_gen_addi_i32(tmp, tmp, offset);
+
+            tmp2 = tcg_temp_new_i32();
+            tcg_gen_movi_i32(tmp2, s->pc | 1);
+            store_reg(s, 14, tmp2);
+            gen_bx(s, tmp);
+        } else {
+            /* 0b1111_0xxx_xxxx_xxxx : BL/BLX prefix */
+            uint32_t uoffset = ((int32_t)insn << 21) >> 9;
+
+            tcg_gen_movi_i32(cpu_R[14], s->pc + 2 + uoffset);
+        }
         break;
     }
     return;
-undef32:
-    gen_exception_insn(s, 4, EXCP_UDEF, syn_uncategorized(),
-                       default_exception_el(s));
-    return;
 illegal_op:
 undef:
     gen_exception_insn(s, 2, EXCP_UDEF, syn_uncategorized(),
@@ -12119,12 +12131,38 @@ static void thumb_tr_translate_insn(DisasContextBase *dcbase, CPUState *cpu)
 {
     DisasContext *dc = container_of(dcbase, DisasContext, base);
     CPUARMState *env = cpu->env_ptr;
+    uint32_t insn;
+    bool is_16bit;
 
     if (arm_pre_translate_insn(dc)) {
         return;
     }
 
-    disas_thumb_insn(env, dc);
+    insn = arm_lduw_code(env, dc->pc, dc->sctlr_b);
+    is_16bit = thumb_insn_is_16bit(dc, insn);
+    dc->pc += 2;
+    if (!is_16bit) {
+        uint32_t insn2 = arm_lduw_code(env, dc->pc, dc->sctlr_b);
+
+        insn = insn << 16 | insn2;
+        dc->pc += 2;
+    }
+
+    if (dc->condexec_mask) {
+        uint32_t cond = dc->condexec_cond;
+
+        if (cond != 0x0e) {     /* Skip conditional when condition is AL. */
+            dc->condlabel = gen_new_label();
+            arm_gen_test_cc(cond ^ 1, dc->condlabel);
+            dc->condjmp = 1;
+        }
+    }
+
+    if (is_16bit) {
+        disas_thumb_insn(dc, insn);
+    } else {
+        disas_thumb2_insn(dc, insn);
+    }
 
     /* Advance the Thumb condexec condition.  */
     if (dc->condexec_mask) {
-- 
2.7.4