cmd/gc: fix x=x crash

[Same as CL 102820043 except applied changes to 6g/gsubr.c
also to 5g/gsubr.c and 8g/gsubr.c. The problem I had last night
trying to do that was that 8g's copy of nodarg has different
(but equivalent) control flow and I was pasting the new code
into the wrong place.]

Description from CL 102820043:

The 'nodarg' function is used to obtain a Node*
representing a function argument or result.
It returned a brand new Node*, but that violates
the guarantee in most places in the compiler that
two Node*s refer to the same variable if and only if
they are the same Node* pointer. Reestablish that
invariant by making nodarg return a preexisting
named variable if present.

Having fixed that, avoid any copy during x=x in
componentgen, because the VARDEF we emit
before the copy marks the lhs x as dead incorrectly.

The change in walk.c avoids modifying the result
of nodarg. This was the only place in the compiler
that did so.

Fixes issue 8097.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, iant, khr, r
https://codereview.appspot.com/103750043

2 files changed, 17 insertions, 0 deletions

diff --git a/src/cmd/5g/cgen.c b/src/cmd/5g/cgen.c
index 9faf75461..9011b2022 100644
--- a/src/cmd/5g/cgen.c
+++ b/src/cmd/5g/cgen.c
@@ -1490,6 +1490,7 @@ sgen(Node *n, Node *res, int64 w)
 	}
 	if(osrc%align != 0 || odst%align != 0)
 		fatal("sgen: unaligned offset src %d or dst %d (align %d)", osrc, odst, align);
+
 	// if we are copying forward on the stack and
 	// the src and dst overlap, then reverse direction
 	dir = align;
@@ -1674,6 +1675,13 @@ componentgen(Node *nr, Node *nl)
 		freer = 1;
 	}
 
+	// nl and nr are 'cadable' which basically means they are names (variables) now.
+	// If they are the same variable, don't generate any code, because the
+	// VARDEF we generate will mark the old value as dead incorrectly.
+	// (And also the assignments are useless.)
+	if(nr != N && nl->op == ONAME && nr->op == ONAME && nl == nr)
+		goto yes;
+
 	switch(nl->type->etype) {
 	case TARRAY:
 		if(nl->op == ONAME)
diff --git a/src/cmd/5g/gsubr.c b/src/cmd/5g/gsubr.c
index 528e8f8cc..f66c87b5a 100644
--- a/src/cmd/5g/gsubr.c
+++ b/src/cmd/5g/gsubr.c
@@ -470,6 +470,7 @@ Node*
 nodarg(Type *t, int fp)
 {
 	Node *n;
+	NodeList *l;
 	Type *first;
 	Iter savet;
 
@@ -491,6 +492,14 @@ nodarg(Type *t, int fp)
 	if(t->etype != TFIELD)
 		fatal("nodarg: not field %T", t);
 
+	if(fp == 1) {
+		for(l=curfn->dcl; l; l=l->next) {
+			n = l->n;
+			if((n->class == PPARAM || n->class == PPARAMOUT) && !isblanksym(t->sym) && n->sym == t->sym)
+				return n;
+		}
+	}
+
 	n = nod(ONAME, N, N);
 	n->type = t->type;
 	n->sym = t->sym;