tcg/riscv: Fix range matched by TCG_CT_CONST_M12

We were matching a signed 13-bit range, not a 12-bit range.
Expand the commentary within the function and be explicit
about all of the ranges.

Reported-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221022095821.2441874-1-richard.henderson@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

1 files changed, 16 insertions, 3 deletions

diff --git a/tcg/riscv/tcg-target.c.inc b/tcg/riscv/tcg-target.c.inc
index 81a83e45b1..191197853f 100644
--- a/tcg/riscv/tcg-target.c.inc
+++ b/tcg/riscv/tcg-target.c.inc
@@ -154,13 +154,26 @@ static bool tcg_target_const_match(int64_t val, TCGType type, int ct)
     if ((ct & TCG_CT_CONST_ZERO) && val == 0) {
         return 1;
     }
-    if ((ct & TCG_CT_CONST_S12) && val == sextreg(val, 0, 12)) {
+    /*
+     * Sign extended from 12 bits: [-0x800, 0x7ff].
+     * Used for most arithmetic, as this is the isa field.
+     */
+    if ((ct & TCG_CT_CONST_S12) && val >= -0x800 && val <= 0x7ff) {
         return 1;
     }
-    if ((ct & TCG_CT_CONST_N12) && -val == sextreg(-val, 0, 12)) {
+    /*
+     * Sign extended from 12 bits, negated: [-0x7ff, 0x800].
+     * Used for subtraction, where a constant must be handled by ADDI.
+     */
+    if ((ct & TCG_CT_CONST_N12) && val >= -0x7ff && val <= 0x800) {
         return 1;
     }
-    if ((ct & TCG_CT_CONST_M12) && val >= -0xfff && val <= 0xfff) {
+    /*
+     * Sign extended from 12 bits, +/- matching: [-0x7ff, 0x7ff].
+     * Used by addsub2, which may need the negative operation,
+     * and requires the modified constant to be representable.
+     */
+    if ((ct & TCG_CT_CONST_M12) && val >= -0x7ff && val <= 0x7ff) {
         return 1;
     }
     return 0;