GCtrace: Make it possible to map between IR and mcode insns

Add a 'szirmcode' array to GCtrace that keeps track of how many bytes
of machine code were assembled for each IR instruction.

This allows us to construct an association between IR code and mcode,
either to see the mcode that was assembled for a given IR instruction,
or to map a machine code instruction back to the relevant IR instruction.

src/lj_asm.c

@@ -1984,6 +1984,10 @@ void lj_asm_trace(jit_State *J, GCtrace *T)
   as->loopinv = 0;
   as->parent = J->parent ? traceref(J, J->parent) : NULL;
 
+  /* Initialize mcode size of IR instructions array. */
+  T->szirmcode = lj_mem_new(J->L, T->nins * sizeof(*T->szirmcode));
+  memset(T->szirmcode, 0, T->nins * sizeof(*T->szirmcode));
+
   /* Reserve MCode memory. */
   as->mctop = origtop = lj_mcode_reserve(J, &as->mcbot);
   as->mcp = as->mctop;
@@ -2043,6 +2047,7 @@ void lj_asm_trace(jit_State *J, GCtrace *T)
     /* Assemble a trace in linear backwards order. */
     for (as->curins--; as->curins > as->stopins; as->curins--) {
       IRIns *ir = IR(as->curins);
+      MCode *end = as->mcp;
       lua_assert(!(LJ_32 && irt_isint64(ir->t)));  /* Handled by SPLIT. */
       if (!ra_used(ir) && !ir_sideeff(ir) && (as->flags & JIT_F_OPT_DCE))
 	continue;  /* Dead-code elimination can be soooo easy. */
@@ -2051,6 +2056,7 @@ void lj_asm_trace(jit_State *J, GCtrace *T)
       RA_DBG_REF();
       checkmclim(as);
       asm_ir(as, ir);
+      T->szirmcode[as->curins] = (uint16_t)(end - as->mcp);
     }
 
     if (as->realign && J->curfinal->nins >= T->nins)

src/lj_jit.h

@@ -192,6 +192,7 @@ typedef struct GCtrace {
   MSize szmcode;	/* Size of machine code. */
   MCode *mcode;		/* Start of machine code. */
   MSize mcloop;		/* Offset of loop start in machine code. */
+  uint16_t *szirmcode;  /* Bytes of mcode for each IR instruction (array.) */
   uint16_t nchild;	/* Number of child traces (root trace only). */
   uint16_t spadjust;	/* Stack pointer adjustment (offset in bytes). */
   TraceNo1 traceno;	/* Trace number. */