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c: use hint for register pair

Avoid register allocation leak on 32-bit to 64-bit casts,
by re-using the target register.

This change allows 5c (and hopefully the other ones) to work with more
complex expressions without running out of fixed registers.
The expected byproduct is a minimal reduction in the number of code
size and therefore speed.

A generic stack spill could be better, but this suffices to compile some
programs that make heavy use of 64-bit arithmetic.

--- a/sys/src/cmd/5c/cgen.c

+++ b/sys/src/cmd/5c/cgen.c

@@ -954,7 +954,7 @@

 					regfree(&nod0);

 					regfree(&nod1);

 				} else {

-					regalloc(&nod0, l, Z);

+					regalloc(&nod0, l, nn);

 					cgen(l, &nod0);

 					cgen(nod0.left, nn);

 					regfree(&nod0);

--- a/sys/src/cmd/5c/txt.c

+++ b/sys/src/cmd/5c/txt.c

@@ -338,7 +338,7 @@

 			regalloc(n->left, &regnode, o->left);

 			regalloc(n->right, &regnode, o->right);

 		} else {

-			regalloc(n->left, &regnode, Z);

+			regalloc(n->left, &regnode, o);

 			regalloc(n->right, &regnode, Z);

 		}

 		if(n->left->reg > n->right->reg){

--- a/sys/src/cmd/8c/txt.c

+++ b/sys/src/cmd/8c/txt.c

@@ -331,7 +331,7 @@

 			regalloc(n->left, &regnode, o->left);

 			regalloc(n->right, &regnode, o->right);

 		} else {

-			regalloc(n->left, &regnode, Z);

+			regalloc(n->left, &regnode, o);

 			regalloc(n->right, &regnode, Z);

 		}

 		n->right->type = types[TULONG];

--- a/sys/src/cmd/qc/txt.c

+++ b/sys/src/cmd/qc/txt.c

@@ -336,7 +336,7 @@

 			regalloc(n->right, &regnode, o->right);

 		} else {

 			regalloc(n->left, &regnode, Z);

-			regalloc(n->right, &regnode, Z);

+			regalloc(n->right, &regnode, o);

 		}

 		n->right->type = types[TULONG];

 		if(tn->type->etype == TUVLONG)

-- 

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