Standardize use of %r7, %r8, and %r9 in trampoline generation

In libdtrace/dt_cg.c, a number of functions generate the trampoline.

After a call to dt_cg_tramp_prologue[_act](), we can expect:
    %r7 contains a pointer to dctx->mst
    %r8 contains a pointer to dctx->ctx
    %r9 contains a pointer to dctx

Most other calls to dt_cg_tramp_*() functions rely on these values.
However,
    dt_cg_tramp_copy_args_from_regs()
    dt_cg_tramp_copy_regs()
    dt_cg_tramp_copy_rval_from_regs()
all pass BPF_REG_8 in via an argument rp.  This is unnecessary.  In fact,
the last two functions don't even use the passed-in argument, already using
the known, hard-wired value BPF_REG_8 instead.

Do not pass BPF_REG_8 in to these functions.  Clean up the comments and
interfaces and switch dt_cg_tramp_copy_args_from_regs() to use BPF_REG_8
explicitly, as the other functions already do.

Signed-off-by: Eugene Loh <eugene.loh@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>

libdtrace/dt_cg.c

@@ -287,20 +287,20 @@ dt_cg_tramp_clear_regs(dt_pcb_t *pcb)
 }
 
 /*
- * Copy the content of a dt_pt_regs structure referenced by the 'rp' argument
- * into the 'regs' member of the machine state.
+ * Copy the content of a dt_pt_regs structure referenced by %r8 into the 'regs'
+ * member of the machine state.
  *
- * The caller must ensure that %r7 contains the value set by the
+ * The caller must ensure that %r7 and %r8 contain the values set by the
  * dt_cg_tramp_prologue*() functions.
  */
 void
-dt_cg_tramp_copy_regs(dt_pcb_t *pcb, int rp)
+dt_cg_tramp_copy_regs(dt_pcb_t *pcb)
 {
 	dt_irlist_t	*dlp = &pcb->pcb_ir;
 	int		i;
 
 	/*
-	 *	dctx->mst->regs = *(dt_pt_regs *)rp;
+	 *	dctx->mst->regs = *(dt_pt_regs *)%r8;
 	 */
 	for (i = 0; i < sizeof(dt_pt_regs); i += 8) {
 		emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, i));
@@ -309,45 +309,45 @@ dt_cg_tramp_copy_regs(dt_pcb_t *pcb, int rp)
 }
 
 /*
- * Copy arguments from a dt_pt_regs structure referenced by the 'rp' argument.
+ * Copy arguments from a dt_pt_regs structure referenced by %r8.
  * If 'called' is nonzero, the registers are laid out as when inside the
  * function: if zero, they are laid out as at the call instruction, before the
  * function is called (as is done for e.g. usdt).
  *
- * The caller must ensure that %r7 contains the value set by the
+ * The caller must ensure that %r7 and %r8 contain the values set by the
  * dt_cg_tramp_prologue*() functions.
  */
 void
-dt_cg_tramp_copy_args_from_regs(dt_pcb_t *pcb, int rp, int called)
+dt_cg_tramp_copy_args_from_regs(dt_pcb_t *pcb, int called)
 {
 	dtrace_hdl_t	*dtp = pcb->pcb_hdl;
 	dt_irlist_t	*dlp = &pcb->pcb_ir;
 	int		i;
 
 	/*
 	 *	for (i = 0; i < PT_REGS_ARGC; i++)
-	 *		dctx->mst->argv[i] = PT_REGS_ARGi((dt_pt_regs *)rp);
-	 *				// lddw %r0, [%rp + PT_REGS_ARGi]
+	 *		dctx->mst->argv[i] = PT_REGS_ARGi((dt_pt_regs *)%r8);
+	 *				// lddw %r0, [%r8 + PT_REGS_ARGi]
 	 *				// stdw [%r7 + DMST_ARG(i)], %r0
 	 */
-	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, rp, PT_REGS_ARG0));
+	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, PT_REGS_ARG0));
 	emit(dlp,  BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(0), BPF_REG_0));
-	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, rp, PT_REGS_ARG1));
+	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, PT_REGS_ARG1));
 	emit(dlp,  BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(1), BPF_REG_0));
-	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, rp, PT_REGS_ARG2));
+	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, PT_REGS_ARG2));
 	emit(dlp,  BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(2), BPF_REG_0));
-	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, rp, PT_REGS_ARG3));
+	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, PT_REGS_ARG3));
 	emit(dlp,  BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(3), BPF_REG_0));
-	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, rp, PT_REGS_ARG4));
+	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, PT_REGS_ARG4));
 	emit(dlp,  BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(4), BPF_REG_0));
-	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, rp, PT_REGS_ARG5));
+	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, PT_REGS_ARG5));
 	emit(dlp,  BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(5), BPF_REG_0));
 #ifdef PT_REGS_ARG6
-	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, rp, PT_REGS_ARG6));
+	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, PT_REGS_ARG6));
 	emit(dlp,  BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(6), BPF_REG_0));
 #endif
 #ifdef PT_REGS_ARG7
-	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, rp, PT_REGS_ARG7));
+	emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, PT_REGS_ARG7));
 	emit(dlp,  BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(7), BPF_REG_0));
 #endif
 
@@ -360,15 +360,15 @@ dt_cg_tramp_copy_args_from_regs(dt_pcb_t *pcb, int rp, int called)
 	 *		int		rc;
 	 *		uint64_t	*sp;
 	 *
-	 *		sp = (uint64_t *)(((dt_pt_regs *)rp)->sp;
+	 *		sp = (uint64_t *)(((dt_pt_regs *)%r8)->sp;
 	 *		rc = bpf_probe_read[_user](dctx->mst->argv[i],
 	 *					   sizeof(uint64_t),
 	 *					   &sp[i - PT_REGS_ARGC +
 	 *						   (called ? PT_REGS_ARGSTKBASE : 0)]);
 	 *				// mov %r1, %r7
 	 *				// add %r1, DMST_ARG(i)
 	 *				// mov %r2, sizeof(uint64_t)
-	 *				// lddw %r3, [%rp + PT_REGS_SP]
+	 *				// lddw %r3, [%r8 + PT_REGS_SP]
 	 *				// add %r3, (i - PT_REGS_ARGC +
 	 *						 (called ? PT_REGS_ARGSTKBASE : 0)) *
 	 *					    sizeof(uint64_t)
@@ -389,7 +389,7 @@ dt_cg_tramp_copy_args_from_regs(dt_pcb_t *pcb, int rp, int called)
 		emit(dlp,  BPF_MOV_REG(BPF_REG_1, BPF_REG_7));
 		emit(dlp,  BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, DMST_ARG(i)));
 		emit(dlp,  BPF_MOV_IMM(BPF_REG_2, sizeof(uint64_t)));
-		emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_3, rp, PT_REGS_SP));
+		emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_3, BPF_REG_8, PT_REGS_SP));
 		emit(dlp,  BPF_ALU64_IMM(BPF_ADD, BPF_REG_3, (i - PT_REGS_ARGC +
 			    (called ? PT_REGS_ARGSTKBASE : 0)) * sizeof(uint64_t)));
 		emit(dlp,  BPF_CALL_HELPER(dtp->dt_bpfhelper[BPF_FUNC_probe_read_user]));
@@ -401,14 +401,14 @@ dt_cg_tramp_copy_args_from_regs(dt_pcb_t *pcb, int rp, int called)
 }
 
 /*
- * Copy return value from a dt_pt_regs structure referenced by the 'rp' argument.
- * to mst->arg[1].  Zero the other args.
+ * Copy return value from a dt_pt_regs structure referenced by %r8 to
+ * mst->arg[1].  Zero the other args.
  *
- * The caller must ensure that %r7 contains the value set by the
+ * The caller must ensure that %r7 and %r8 contain the values set by the
  * dt_cg_tramp_prologue*() functions.
  */
 void
-dt_cg_tramp_copy_rval_from_regs(dt_pcb_t *pcb, int rp)
+dt_cg_tramp_copy_rval_from_regs(dt_pcb_t *pcb)
 {
 	dt_irlist_t	*dlp = &pcb->pcb_ir;
 	int		i;

libdtrace/dt_cg.h

@@ -23,9 +23,9 @@ extern dt_irnode_t *dt_cg_node_alloc(uint_t, struct bpf_insn);
 extern void dt_cg_tramp_prologue_act(dt_pcb_t *pcb, dt_activity_t act);
 extern void dt_cg_tramp_prologue(dt_pcb_t *pcb);
 extern void dt_cg_tramp_clear_regs(dt_pcb_t *pcb);
-extern void dt_cg_tramp_copy_regs(dt_pcb_t *pcb, int rp);
-extern void dt_cg_tramp_copy_args_from_regs(dt_pcb_t *pcb, int rp, int called);
-extern void dt_cg_tramp_copy_rval_from_regs(dt_pcb_t *pcb, int rp);
+extern void dt_cg_tramp_copy_regs(dt_pcb_t *pcb);
+extern void dt_cg_tramp_copy_args_from_regs(dt_pcb_t *pcb, int called);
+extern void dt_cg_tramp_copy_rval_from_regs(dt_pcb_t *pcb);
 extern void dt_cg_tramp_call_clauses(dt_pcb_t *pcb, const dt_probe_t *prp,
 				     dt_activity_t act);
 extern void dt_cg_tramp_return(dt_pcb_t *pcb);

libdtrace/dt_prov_dtrace.c

@@ -156,7 +156,7 @@ static void trampoline(dt_pcb_t *pcb)
 	emit(dlp, BPF_MOV_IMM(BPF_REG_4, BPF_ANY));
 	emit(dlp, BPF_CALL_HELPER(BPF_FUNC_map_update_elem));
 
-	dt_cg_tramp_copy_regs(pcb, BPF_REG_8);
+	dt_cg_tramp_copy_regs(pcb);
 
 	/* zero the probe args */
 	for (i = 0; i < ARRAY_SIZE(((dt_mstate_t *)0)->argv); i++)

libdtrace/dt_prov_fbt.c

@@ -157,11 +157,11 @@ static void trampoline(dt_pcb_t *pcb)
 	 *				//     (%r7 = dctx->mst)
 	 *				//     (%r8 = dctx->ctx)
 	 */
-	dt_cg_tramp_copy_regs(pcb, BPF_REG_8);
+	dt_cg_tramp_copy_regs(pcb);
 	if (strcmp(pcb->pcb_probe->desc->prb, "return") == 0) {
 		dt_irlist_t *dlp = &pcb->pcb_ir;
 
-		dt_cg_tramp_copy_rval_from_regs(pcb, BPF_REG_8);
+		dt_cg_tramp_copy_rval_from_regs(pcb);
 
 		/*
 		 * fbt:::return arg0 should be the function offset for
@@ -175,7 +175,7 @@ static void trampoline(dt_pcb_t *pcb)
 		dt_cg_xsetx(dlp, NULL, DT_LBL_NONE, BPF_REG_0, -1);
 		emit(dlp,  BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(0), BPF_REG_0));
 	} else
-		dt_cg_tramp_copy_args_from_regs(pcb, BPF_REG_8, 1);
+		dt_cg_tramp_copy_args_from_regs(pcb, 1);
 	dt_cg_tramp_epilogue(pcb);
 }
 

libdtrace/dt_prov_profile.c

@@ -226,7 +226,7 @@ static void trampoline(dt_pcb_t *pcb)
 	 *				//     (%r8 = dctx->ctx)
 	 */
 
-	dt_cg_tramp_copy_regs(pcb, BPF_REG_8);
+	dt_cg_tramp_copy_regs(pcb);
 
 	/*
 	 * TODO:

libdtrace/dt_prov_uprobe.c

@@ -360,11 +360,11 @@ static void trampoline(dt_pcb_t *pcb)
 	 *				//     (%r8 = dctx->ctx)
 	 */
 
-	dt_cg_tramp_copy_regs(pcb, BPF_REG_8);
+	dt_cg_tramp_copy_regs(pcb);
 	if (upp->flags & PP_IS_RETURN)
-		dt_cg_tramp_copy_rval_from_regs(pcb, BPF_REG_8);
+		dt_cg_tramp_copy_rval_from_regs(pcb);
 	else
-		dt_cg_tramp_copy_args_from_regs(pcb, BPF_REG_8,
+		dt_cg_tramp_copy_args_from_regs(pcb,
 						!(upp->flags & PP_IS_FUNCALL));
 
 	/*