cmd/5g, cmd/6g: fix out of registers with array indexing.

Compiling expressions like:
    s[s[s[s[s[s[s[s[s[s[s[s[i]]]]]]]]]]]]
make 5g and 6g run out of registers. Such expressions can arise
if a slice is used to represent a permutation and the user wants
to iterate it.

This is due to the usual problem of allocating registers before
going down the expression tree, instead of allocating them in a
postfix way.

The functions cgenr and agenr (that generate a value to a newly
allocated register instead of an existing location), are either
introduced or modified when they already existed to allocate
the new register as late as possible, and sudoaddable is disabled
for OINDEX nodes so that igen/agenr is used instead.

Update issue 4207.

R=dave, daniel.morsing, rsc
CC=golang-dev
http://codereview.appspot.com/6733055

1 files changed, 17 insertions, 0 deletions

diff --git a/test/torture.go b/test/torture.go
index 4bce3a179..c510bb923 100644
--- a/test/torture.go
+++ b/test/torture.go
@@ -116,6 +116,23 @@ func determinantByte(m [4][4]byte) byte {
 		m[0][3]*m[1][2]*m[2][1]*m[3][0]
 }
 
+type A []A
+
+// A sequence of constant indexings.
+func IndexChain1(s A) A {
+	return s[0][0][0][0][0][0][0][0][0][0][0][0][0][0][0][0]
+}
+
+// A sequence of non-constant indexings.
+func IndexChain2(s A, i int) A {
+	return s[i][i][i][i][i][i][i][i][i][i][i][i][i][i][i][i]
+}
+
+// Another sequence of indexings.
+func IndexChain3(s []int) int {
+	return s[s[s[s[s[s[s[s[s[s[s[s[s[s[s[s[s[s[s[s[s[0]]]]]]]]]]]]]]]]]]]]]
+}
+
 // A right-leaning tree of byte multiplications.
 func righttree(a, b, c, d uint8) uint8 {
 	return a * (b * (c * (d *