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, 0 deletions

diff --git a/py/asmbase.c b/py/asmbase.c
index 4a3fd089c..da4273506 100644
--- a/py/asmbase.c
+++ b/py/asmbase.c
@@ -55,15 +55,20 @@ void mp_asm_base_start_pass(mp_asm_base_t *as, int pass) {
         assert(as->code_base != NULL);
     }
     as->pass = pass;
+    as->suppress = false;
     as->code_offset = 0;
 }
 
 // all functions must go through this one to emit bytes
 // if as->pass < MP_ASM_PASS_EMIT, then this function just counts the number
 // of bytes needed and returns NULL, and callers should not store any data
+// It also returns NULL if generated code should be suppressed at this point.
 uint8_t *mp_asm_base_get_cur_to_write_bytes(void *as_in, size_t num_bytes_to_write) {
     mp_asm_base_t *as = as_in;
     uint8_t *c = NULL;
+    if (as->suppress) {
+        return c;
+    }
     if (as->pass == MP_ASM_PASS_EMIT) {
         assert(as->code_offset + num_bytes_to_write <= as->code_size);
         c = as->code_base + as->code_offset;
@@ -74,6 +79,11 @@ uint8_t *mp_asm_base_get_cur_to_write_bytes(void *as_in, size_t num_bytes_to_wri
 
 void mp_asm_base_label_assign(mp_asm_base_t *as, size_t label) {
     assert(label < as->max_num_labels);
+
+    // Assiging a label ends any dead-code region, and all following machine
+    // code should be emitted (until another mp_asm_base_suppress_code() call).
+    as->suppress = false;
+
     if (as->pass < MP_ASM_PASS_EMIT) {
         // assign label offset
         assert(as->label_offsets[label] == (size_t)-1);