[DTrace-devel] [PATCH 2/2] Optimize dt_cg_typecast() since shift amount is known

[eugene.loh at oracle.com]

From: Eugene Loh <eugene.loh at oracle.com>

The typecast consists of shifting left and then right.  Since the shift
amount is known at the time of code generation, some optimizations are
possible.

Specifically, use IMM instructions to shift, thereby saving a register
and the instruction needed to load it.

Further, if the shift amount is zero, generate no shift instructions
at all.  This change happens to eliminate the BPF error encountered in
test/unittest/variables/bvar/tst.curcpu.d so that it now passes.

Signed-off-by: Eugene Loh <eugene.loh at oracle.com>
Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
---
 libdtrace/dt_cg.c                         | 28 ++++++++---------------
 test/unittest/variables/bvar/tst.curcpu.d |  1 -
 2 files changed, 10 insertions(+), 19 deletions(-)

diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
index b9052b26..0d3463c1 100644
--- a/libdtrace/dt_cg.c
+++ b/libdtrace/dt_cg.c
@@ -1488,7 +1488,7 @@ dt_cg_typecast(const dt_node_t *src, const dt_node_t *dst,
 	size_t srcsize;
 	size_t dstsize;
 	struct bpf_insn instr;
-	int reg, n;
+	int n;
 
 	/* If the destination type is '@' (any type) we need not cast. */
 	if (dst->dn_ctfp == NULL && dst->dn_type == CTF_ERR)
@@ -1497,28 +1497,20 @@ dt_cg_typecast(const dt_node_t *src, const dt_node_t *dst,
 	srcsize = dt_node_type_size(src);
 	dstsize = dt_node_type_size(dst);
 
-	if (dt_node_is_scalar(dst) && (dstsize < srcsize ||
-	    (src->dn_flags & DT_NF_SIGNED) ^ (dst->dn_flags & DT_NF_SIGNED))) {
-		if ((reg = dt_regset_alloc(drp)) == -1)
-			longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
-
-		if (dstsize < srcsize)
-			n = sizeof (uint64_t) * NBBY - dstsize * NBBY;
-		else
-			n = sizeof (uint64_t) * NBBY - srcsize * NBBY;
-
-		dt_cg_setx(dlp, reg, n);
+	if (dstsize < srcsize)
+		n = sizeof (uint64_t) * NBBY - dstsize * NBBY;
+	else
+		n = sizeof (uint64_t) * NBBY - srcsize * NBBY;
 
+	if (dt_node_is_scalar(dst) && n != 0 && (dstsize < srcsize ||
+	    (src->dn_flags & DT_NF_SIGNED) ^ (dst->dn_flags & DT_NF_SIGNED))) {
 		instr = BPF_MOV_REG(dst->dn_reg, src->dn_reg);
 		dt_irlist_append(dlp, dt_cg_node_alloc(DT_LBL_NONE, instr));
-		instr = BPF_ALU64_REG(BPF_LSH, dst->dn_reg, reg);
+		instr = BPF_ALU64_IMM(BPF_LSH, dst->dn_reg, n);
 		dt_irlist_append(dlp, dt_cg_node_alloc(DT_LBL_NONE, instr));
-
-		instr = BPF_ALU64_REG((dst->dn_flags & DT_NF_SIGNED) ?
-		    BPF_ARSH : BPF_RSH, dst->dn_reg, reg);
-
+		instr = BPF_ALU64_IMM((dst->dn_flags & DT_NF_SIGNED) ?
+		    BPF_ARSH : BPF_RSH, dst->dn_reg, n);
 		dt_irlist_append(dlp, dt_cg_node_alloc(DT_LBL_NONE, instr));
-		dt_regset_free(drp, reg);
 	}
 }
 
diff --git a/test/unittest/variables/bvar/tst.curcpu.d b/test/unittest/variables/bvar/tst.curcpu.d
index 7e232e09..e0e266ff 100644
--- a/test/unittest/variables/bvar/tst.curcpu.d
+++ b/test/unittest/variables/bvar/tst.curcpu.d
@@ -4,7 +4,6 @@
  * Licensed under the Universal Permissive License v 1.0 as shown at
  * http://oss.oracle.com/licenses/upl.
  */
-/* @@xfail: dtv2 */
 
 /*
  * ASSERTION: The 'curcpu' variable can be accessed and is not -1.
-- 
2.18.4