cmd/compile: merge zero constant ISEL in PPC64 lateLower pass

Add a new SSA opcode ISELZ, similar to ISELB to represent a select
of value or 0. Then, merge candidate ISEL opcodes inside the late
lower pass.

This avoids complicating rules within the the lower pass.

Change-Id: I3b14c94b763863aadc834b0e910a85870c131313
Reviewed-on: https://go-review.googlesource.com/c/go/+/442596
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Joedian Reid <joedian@golang.org>

1 files changed, 24 insertions, 0 deletions

diff --git a/test/codegen/condmove.go b/test/codegen/condmove.go
index bfab62213b..7b0f32e708 100644
--- a/test/codegen/condmove.go
+++ b/test/codegen/condmove.go
@@ -440,3 +440,27 @@ func cmovzero2(c bool) int {
 	// loong64:"MASKNEZ", -"MASKEQZ"
 	return x
 }
+
+// Conditionally selecting between a value or 0 can be done without
+// an extra load of 0 to a register on PPC64 by using R0 (which always
+// holds the value $0) instead. Verify both cases where either arg1
+// or arg2 is zero.
+func cmovzeroreg0(a, b int) int {
+	x := 0
+	if a == b {
+		x = a
+	}
+	// ppc64:"ISEL\t[$]2, R[0-9]+, R0, R[0-9]+"
+	// ppc64le:"ISEL\t[$]2, R[0-9]+, R0, R[0-9]+"
+	return x
+}
+
+func cmovzeroreg1(a, b int) int {
+	x := a
+	if a == b {
+		x = 0
+	}
+	// ppc64:"ISEL\t[$]2, R0, R[0-9]+, R[0-9]+"
+	// ppc64le:"ISEL\t[$]2, R0, R[0-9]+, R[0-9]+"
+	return x
+}