From 6838496e5c990339755577eb16b949fe2f47790e Mon Sep 17 00:00:00 2001
From: Michael Forney <mforney@mforney.org>
Date: Tue, 2 Jul 2019 20:32:09 -0700
Subject: fold: Don't fold invalid addition/subtraction rather than failing

This may happen in a branch QBE doesn't realize is unreachable,
for example (simplified from real code found in ncurses)

	data $str = { b "abcdef", b 0 }
	function l $f(w %x) {
	@start
		%.1 =w ceqw %x, 0
		jnz %.1, @logic_join, @logic_right
	@logic_right
		%p =l call $strchr(l $str, w %x)
		%.2 =w ceql %p, 0
	@logic_join
		%.3 =w phi @start %.1, @logic_right %.2
		jnz %.3, @fail, @return
	@fail
		ret 0
	@return
		%.4 =l sub %p, $str
		ret %.4
	}
---
 fold.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fold.c b/fold.c
index 2081a72..5c09469 100644
--- a/fold.c
+++ b/fold.c
@@ -343,7 +343,7 @@ foldint(Con *res, int op, int w, Con *cl, Con *cr)
 	if (op == Oadd) {
 		if (cl->type == CAddr) {
 			if (cr->type == CAddr)
-				err("undefined addition (addr + addr)");
+				return 1;
 			lab = cl->label;
 			typ = CAddr;
 		}
@@ -358,10 +358,10 @@ foldint(Con *res, int op, int w, Con *cl, Con *cr)
 				lab = cl->label;
 				typ = CAddr;
 			} else if (cl->label != cr->label)
-				err("undefined substraction (addr1 - addr2)");
+				return 1;
 		}
 		else if (cr->type == CAddr)
-			err("undefined substraction (num - addr)");
+			return 1;
 	}
 	else if (cl->type == CAddr || cr->type == CAddr) {
 		if (Ocmpl <= op && op <= Ocmpl1)
-- 
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