linux-user/host/riscv: Improve host_signal_write

Do not read 4 bytes before we determine the size of the insn.
Simplify triple switches in favor of checking major opcodes.
Include the missing cases of compact fsd and fsdsp.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/host/riscv/host-signal.h

@@ -18,65 +18,38 @@ static inline uintptr_t host_signal_pc(ucontext_t *uc)
 
 static inline bool host_signal_write(siginfo_t *info, ucontext_t *uc)
 {
-    uint32_t insn = *(uint32_t *)host_signal_pc(uc);
-
     /*
-     * Detect store by reading the instruction at the program
-     * counter. Note: we currently only generate 32-bit
-     * instructions so we thus only detect 32-bit stores
+     * Detect store by reading the instruction at the program counter.
+     * Do not read more than 16 bits, because we have not yet determined
+     * the size of the instruction.
      */
-    switch (((insn >> 0) & 0b11)) {
-    case 3:
-        switch (((insn >> 2) & 0b11111)) {
-        case 8:
-            switch (((insn >> 12) & 0b111)) {
-            case 0: /* sb */
-            case 1: /* sh */
-            case 2: /* sw */
-            case 3: /* sd */
-            case 4: /* sq */
-                return true;
-            default:
-                break;
-            }
-            break;
-        case 9:
-            switch (((insn >> 12) & 0b111)) {
-            case 2: /* fsw */
-            case 3: /* fsd */
-            case 4: /* fsq */
-                return true;
-            default:
-                break;
-            }
-            break;
-        default:
-            break;
-        }
+    const uint16_t *pinsn = (const uint16_t *)host_signal_pc(uc);
+    uint16_t insn = pinsn[0];
+
+    /* 16-bit instructions */
+    switch (insn & 0xe003) {
+    case 0xa000: /* c.fsd */
+    case 0xc000: /* c.sw */
+    case 0xe000: /* c.sd (rv64) / c.fsw (rv32) */
+    case 0xa002: /* c.fsdsp */
+    case 0xc002: /* c.swsp */
+    case 0xe002: /* c.sdsp (rv64) / c.fswsp (rv32) */
+        return true;
     }
 
-    /* Check for compressed instructions */
-    switch (((insn >> 13) & 0b111)) {
-    case 7:
-        switch (insn & 0b11) {
-        case 0: /*c.sd */
-        case 2: /* c.sdsp */
-            return true;
-        default:
-            break;
-        }
-        break;
-    case 6:
-        switch (insn & 0b11) {
-        case 0: /* c.sw */
-        case 3: /* c.swsp */
-            return true;
-        default:
-            break;
-        }
-        break;
-    default:
-        break;
+    /* 32-bit instructions, major opcodes */
+    switch (insn & 0x7f) {
+    case 0x23: /* store */
+    case 0x27: /* store-fp */
+        return true;
+    case 0x2f: /* amo */
+        /*
+         * The AMO function code is in bits 25-31, unread as yet.
+         * The AMO functions are LR (read), SC (write), and the
+         * rest are all read-modify-write.
+         */
+        insn = pinsn[1];
+        return (insn >> 11) != 2; /* LR */
     }
 
     return false;