py/emit: Suppress unreachable bytecode/native code that follows jump.

This new logic tracks when an unconditional jump/raise occurs in the
emitted code stream (bytecode or native machine code) and suppresses all
subsequent code, until a label is assigned.  This eliminates a lot of
cases of dead code, with relatively simple logic.

This commit combined with the previous one (that removed the existing
dead-code finding logic) has the following code size change:

       bare-arm:   -16 -0.028%
    minimal x86:   -60 -0.036%
       unix x64:  -368 -0.070%
    unix nanbox:   -80 -0.017%
          stm32:  -204 -0.052% PYBV10
         cc3200:    +0 +0.000%
        esp8266:  -232 -0.033% GENERIC
          esp32:  -224 -0.015% GENERIC[incl -40(data)]
         mimxrt:  -192 -0.054% TEENSY40
     renesas-ra:  -200 -0.032% RA6M2_EK
            nrf:   +28 +0.015% pca10040
            rp2:  -256 -0.050% PICO
           samd:   -12 -0.009% ADAFRUIT_ITSYBITSY_M4_EXPRESS

Signed-off-by: Damien George <damien@micropython.org>

1 files changed, 10 insertions, 1 deletions

diff --git a/py/asmbase.h b/py/asmbase.h
index 960be7685..352d2f54c 100644
--- a/py/asmbase.h
+++ b/py/asmbase.h
@@ -33,7 +33,12 @@
 #define MP_ASM_PASS_EMIT    (2)
 
 typedef struct _mp_asm_base_t {
-    int pass;
+    uint8_t pass;
+
+    // Set to true using mp_asm_base_suppress_code() if the code generator
+    // should suppress emitted code due to it being dead code.
+    bool suppress;
+
     size_t code_offset;
     size_t code_size;
     uint8_t *code_base;
@@ -50,6 +55,10 @@ void mp_asm_base_label_assign(mp_asm_base_t *as, size_t label);
 void mp_asm_base_align(mp_asm_base_t *as, unsigned int align);
 void mp_asm_base_data(mp_asm_base_t *as, unsigned int bytesize, uintptr_t val);
 
+static inline void mp_asm_base_suppress_code(mp_asm_base_t *as) {
+    as->suppress = true;
+}
+
 static inline size_t mp_asm_base_get_code_pos(mp_asm_base_t *as) {
     return as->code_offset;
 }