py/emitnative: Emit shorter exception handler entry code on RV32.

This commit improves the RV32 code sequence that is emitted if a
function needs to set up an exception handler as its prologue.

The old code would clear a temporary register and then copy that value
to places that needed to be initialised with zero values.  On RV32
there's a dedicated register that's hardwired to be equal to zero, which
allows us to bypass the extra register clear and use the zero register
to initialise values.

Signed-off-by: Alessandro Gatti <[email protected]>

Author: agatti

py/asmrv32.h

@@ -694,6 +694,7 @@ static inline void asm_rv32_opcode_xori(asm_rv32_t *state, mp_uint_t rd, mp_uint
 #define REG_LOCAL_1 ASM_RV32_REG_S3
 #define REG_LOCAL_2 ASM_RV32_REG_S4
 #define REG_LOCAL_3 ASM_RV32_REG_S5
+#define REG_ZERO ASM_RV32_REG_ZERO
 
 void asm_rv32_meta_comparison_eq(asm_rv32_t *state, mp_uint_t rs1, mp_uint_t rs2, mp_uint_t rd);
 void asm_rv32_meta_comparison_ne(asm_rv32_t *state, mp_uint_t rs1, mp_uint_t rs2, mp_uint_t rd);
@@ -756,6 +757,7 @@ void asm_rv32_emit_store_reg_reg_offset(asm_rv32_t *state, mp_uint_t source, mp_
 #define ASM_STORE_REG_REG(state, rs1, rs2) ASM_STORE32_REG_REG(state, rs1, rs2)
 #define ASM_SUB_REG_REG(state, rd, rs) asm_rv32_opcode_sub(state, rd, rd, rs)
 #define ASM_XOR_REG_REG(state, rd, rs) asm_rv32_emit_optimised_xor(state, rd, rs)
+#define ASM_CLR_REG(state, rd)
 
 #endif
 

py/emitnative.c

@@ -288,6 +288,11 @@ struct _emit_t {
     ASM_T *as;
 };
 
+#ifndef REG_ZERO
+#define REG_ZERO REG_TEMP0
+#define ASM_CLR_REG(state, rd) ASM_XOR_REG_REG(state, rd, rd)
+#endif
+
 static void emit_load_reg_with_object(emit_t *emit, int reg, mp_obj_t obj);
 static void emit_native_global_exc_entry(emit_t *emit);
 static void emit_native_global_exc_exit(emit_t *emit);
@@ -1200,12 +1205,12 @@ static void emit_native_global_exc_entry(emit_t *emit) {
             ASM_JUMP_IF_REG_ZERO(emit->as, REG_RET, start_label, true);
         } else {
             // Clear the unwind state
-            ASM_XOR_REG_REG(emit->as, REG_TEMP0, REG_TEMP0);
-            ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_EXC_HANDLER_UNWIND(emit), REG_TEMP0);
+            ASM_CLR_REG(emit->as, REG_ZERO);
+            ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_EXC_HANDLER_UNWIND(emit), REG_ZERO);
 
             // clear nlr.ret_val, because it's passed to mp_native_raise regardless
             // of whether there was an exception or not
-            ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_EXC_VAL(emit), REG_TEMP0);
+            ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_EXC_VAL(emit), REG_ZERO);
 
             // Put PC of start code block into REG_LOCAL_1
             ASM_MOV_REG_PCREL(emit->as, REG_LOCAL_1, start_label);
@@ -1221,8 +1226,8 @@ static void emit_native_global_exc_entry(emit_t *emit) {
             ASM_JUMP_IF_REG_NONZERO(emit->as, REG_RET, global_except_label, true);
 
             // Clear PC of current code block, and jump there to resume execution
-            ASM_XOR_REG_REG(emit->as, REG_TEMP0, REG_TEMP0);
-            ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_EXC_HANDLER_PC(emit), REG_TEMP0);
+            ASM_CLR_REG(emit->as, REG_ZERO);
+            ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_EXC_HANDLER_PC(emit), REG_ZERO);
             ASM_JUMP_REG(emit->as, REG_LOCAL_1);
 
             // Global exception handler: check for valid exception handler