[DTrace-devel] [PATCH] Optimize dt_cg_store_val() for string values

[Kris Van Hees]

Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
---
 libdtrace/dt_cg.c | 48 +++++++++++++----------------------------------
 1 file changed, 13 insertions(+), 35 deletions(-)

diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
index 455e5440..930355f8 100644
--- a/libdtrace/dt_cg.c
+++ b/libdtrace/dt_cg.c
@@ -958,9 +958,6 @@ dt_cg_store_val(dt_pcb_t *pcb, dt_node_t *dnp, dtrace_actkind_t kind,
 
 		return 0;
 	} else if (dt_node_is_string(dnp)) {
-		uint_t		size_ok = dt_irlist_label(dlp);
-		int		reg;
-
 		dt_cg_check_notnull(dlp, drp, dnp->dn_reg);
 
 		TRACE_REGSET("store_val(): Begin ");
@@ -968,49 +965,30 @@ dt_cg_store_val(dt_pcb_t *pcb, dt_node_t *dnp, dtrace_actkind_t kind,
 				 size + DT_STRLEN_BYTES + 1, 1, pfp, arg);
 
 		/*
-		 * Retrieve the length of the string, limit it to the maximum
-		 * string size, and store it in the buffer at [%r9 + off].
+		 * Copy the length of the string from the source string as a
+		 * half-word (2 bytes) into the buffer at [%r9 + off].
 		 */
-		reg = dt_regset_alloc(drp);
-		if (reg == -1)
-			longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
-		dt_cg_strlen(dlp, drp, reg, dnp->dn_reg);
-		dt_regset_xalloc(drp, BPF_REG_0);
-		emit(dlp,  BPF_BRANCH_IMM(BPF_JLT, reg, size, size_ok));
-		emit(dlp,  BPF_MOV_IMM(reg, size));
-		emitl(dlp, size_ok,
-			   BPF_MOV_REG(BPF_REG_0, reg));
-		emit(dlp,  BPF_ALU64_IMM(BPF_RSH, BPF_REG_0, 8));
-		emit(dlp,  BPF_STORE(BPF_B, BPF_REG_9, off, BPF_REG_0));
-		emit(dlp,  BPF_STORE(BPF_B, BPF_REG_9, off + 1, reg));
-		dt_regset_free(drp, BPF_REG_0);
+		emit(dlp, BPF_LOAD(BPF_H, BPF_REG_0, dnp->dn_reg, 0));
+		emit(dlp, BPF_STORE(BPF_H, BPF_REG_9, off, BPF_REG_0));
 
 		/*
-		 * Copy the string to the output buffer at
-		 * [%r9 + off + DT_STRLEN_BYTES] because we need to skip the
-		 * length prefix.
+		 * Copy the string data (no more than STRSIZE + 1 bytes) to the
+		 * buffer at [%r9 + off + DT_STRLEN_BYTES].  We (ab)use the
+		 * fact that probe_read_str) stops at the terminating NUL byte.
 		 */
 		if (dt_regset_xalloc_args(drp) == -1)
 			longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
 
-		emit(dlp,  BPF_MOV_REG(BPF_REG_1, BPF_REG_9));
-		emit(dlp,  BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, off + DT_STRLEN_BYTES));
-		emit(dlp,  BPF_MOV_REG(BPF_REG_2, reg));
-		dt_regset_free(drp, reg);
-		emit(dlp,  BPF_MOV_REG(BPF_REG_3, dnp->dn_reg));
+		emit(dlp, BPF_MOV_REG(BPF_REG_1, BPF_REG_9));
+		emit(dlp, BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, off + DT_STRLEN_BYTES));
+		emit(dlp, BPF_MOV_IMM(BPF_REG_2, size + 1));
+		emit(dlp, BPF_MOV_REG(BPF_REG_3, dnp->dn_reg));
 		dt_regset_free(drp, dnp->dn_reg);
 		dt_cg_tstring_free(pcb, dnp);
-		emit(dlp,  BPF_ALU64_IMM(BPF_ADD, BPF_REG_3, DT_STRLEN_BYTES));
+		emit(dlp, BPF_ALU64_IMM(BPF_ADD, BPF_REG_3, DT_STRLEN_BYTES));
 		dt_regset_xalloc(drp, BPF_REG_0);
-		emit(dlp,  BPF_CALL_HELPER(BPF_FUNC_probe_read));
+		emit(dlp, BPF_CALL_HELPER(BPF_FUNC_probe_read_str));
 		dt_regset_free_args(drp);
-
-		/*
-		 * Write the NUL terminating byte.
-		 */
-		emit(dlp,  BPF_MOV_REG(BPF_REG_0, BPF_REG_9));
-		emit(dlp,  BPF_ALU64_REG(BPF_ADD, BPF_REG_0, reg));
-		emit(dlp,  BPF_STORE_IMM(BPF_B, BPF_REG_0, off + DT_STRLEN_BYTES, 0));
 		dt_regset_free(drp, BPF_REG_0);
 		TRACE_REGSET("store_val(): End   ");
 
-- 
2.33.0