tcg: Introduce the 'z' constraint for a hardware zero register

For loongarch, mips, riscv and sparc, a zero register is
available all the time.  For aarch64, register index 31
depends on context: sometimes it is the stack pointer,
and sometimes it is the zero register.

Introduce a new general-purpose constraint which maps 0
to TCG_REG_ZERO, if defined.  This differs from existing
constant constraints in that const_arg[*] is recorded as
false, indicating that the value is in a register.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

docs/devel/tcg-ops.rst

@@ -927,7 +927,9 @@ operation uses a constant input constraint which does not allow all
 constants, it must also accept registers in order to have a fallback.
 The constraint '``i``' is defined generically to accept any constant.
 The constraint '``r``' is not defined generically, but is consistently
-used by each backend to indicate all registers.
+used by each backend to indicate all registers.  If ``TCG_REG_ZERO``
+is defined by the backend, the constraint '``z``' is defined generically
+to map constant 0 to the hardware zero register.
 
 The movi_i32 and movi_i64 operations must accept any constants.
 

include/tcg/tcg.h

@@ -713,7 +713,8 @@ void tb_target_set_jmp_target(const TranslationBlock *, int,
 
 void tcg_set_frame(TCGContext *s, TCGReg reg, intptr_t start, intptr_t size);
 
-#define TCG_CT_CONST  1 /* any constant of register size */
+#define TCG_CT_CONST      1  /* any constant of register size */
+#define TCG_CT_REG_ZERO   2  /* zero, in TCG_REG_ZERO */
 
 typedef struct TCGArgConstraint {
     unsigned ct : 16;

tcg/aarch64/tcg-target.h

@@ -45,6 +45,8 @@ typedef enum {
     TCG_AREG0  = TCG_REG_X19,
 } TCGReg;
 
+#define TCG_REG_ZERO TCG_REG_XZR
+
 #define TCG_TARGET_NB_REGS 64
 
 #endif /* AARCH64_TCG_TARGET_H */

tcg/loongarch64/tcg-target.h

@@ -85,4 +85,6 @@ typedef enum {
     TCG_VEC_TMP0 = TCG_REG_V23,
 } TCGReg;
 
+#define TCG_REG_ZERO  TCG_REG_ZERO
+
 #endif /* LOONGARCH_TCG_TARGET_H */

tcg/mips/tcg-target.h

@@ -70,4 +70,6 @@ typedef enum {
     TCG_AREG0 = TCG_REG_S8,
 } TCGReg;
 
+#define TCG_REG_ZERO  TCG_REG_ZERO
+
 #endif

tcg/riscv/tcg-target.h

@@ -57,4 +57,6 @@ typedef enum {
     TCG_REG_TMP2       = TCG_REG_T4,
 } TCGReg;
 
+#define TCG_REG_ZERO  TCG_REG_ZERO
+
 #endif

tcg/sparc64/tcg-target.h

@@ -64,6 +64,7 @@ typedef enum {
     TCG_REG_I7,
 } TCGReg;
 
-#define TCG_AREG0 TCG_REG_I0
+#define TCG_AREG0     TCG_REG_I0
+#define TCG_REG_ZERO  TCG_REG_G0
 
 #endif

tcg/tcg.c

@@ -3223,6 +3223,11 @@ static void process_constraint_sets(void)
                 case 'i':
                     args_ct[i].ct |= TCG_CT_CONST;
                     break;
+#ifdef TCG_REG_ZERO
+                case 'z':
+                    args_ct[i].ct |= TCG_CT_REG_ZERO;
+                    break;
+#endif
 
                 /* Include all of the target-specific constraints. */
 
@@ -5074,13 +5079,23 @@ static void tcg_reg_alloc_op(TCGContext *s, const TCGOp *op)
         arg_ct = &args_ct[i];
         ts = arg_temp(arg);
 
-        if (ts->val_type == TEMP_VAL_CONST
-            && tcg_target_const_match(ts->val, arg_ct->ct, ts->type,
-                                      op_cond, TCGOP_VECE(op))) {
-            /* constant is OK for instruction */
-            const_args[i] = 1;
-            new_args[i] = ts->val;
-            continue;
+        if (ts->val_type == TEMP_VAL_CONST) {
+#ifdef TCG_REG_ZERO
+            if (ts->val == 0 && (arg_ct->ct & TCG_CT_REG_ZERO)) {
+                /* Hardware zero register: indicate register via non-const. */
+                const_args[i] = 0;
+                new_args[i] = TCG_REG_ZERO;
+                continue;
+            }
+#endif
+
+            if (tcg_target_const_match(ts->val, arg_ct->ct, ts->type,
+                                       op_cond, TCGOP_VECE(op))) {
+                /* constant is OK for instruction */
+                const_args[i] = 1;
+                new_args[i] = ts->val;
+                continue;
+            }
         }
 
         reg = ts->reg;