From 22338bdfd0346e85a0e88ca5470302a4775e9054 Mon Sep 17 00:00:00 2001
From: Michael Forney <mforney@mforney.org>
Date: Thu, 14 Feb 2019 01:18:40 -0800
Subject: When subtracting pointers, divide after subtraction

The pointers might be global addresses, and it doesn't make sense to
divide them before subtracting.
---
 expr.c                     | 6 +++---
 tests/subtract-pointer.qbe | 9 ++++-----
 2 files changed, 7 insertions(+), 8 deletions(-)

diff --git a/expr.c b/expr.c
index 40162f5..75c0423 100644
--- a/expr.c
+++ b/expr.c
@@ -327,9 +327,9 @@ mkbinaryexpr(struct location *loc, enum tokenkind op, struct expression *l, stru
 			t = l->type;
 			r = mkbinaryexpr(loc, TMUL, exprconvert(r, &typeulong), mkconstexpr(&typeulong, t->base->size));
 		} else if (l->type->kind == TYPEPOINTER && r->type->kind == TYPEPOINTER && typecompatible(typeunqual(l->type->base, NULL), typeunqual(r->type->base, NULL))) {
-			l = mkbinaryexpr(loc, TDIV, exprconvert(l, &typeulong), mkconstexpr(&typeulong, l->type->base->size));
-			r = mkbinaryexpr(loc, TDIV, exprconvert(r, &typeulong), mkconstexpr(&typeulong, r->type->base->size));
-			t = &typelong;
+			/* XXX: when is the appropriate place to convert to signed integer */
+			e = mkbinaryexpr(loc, TSUB, exprconvert(l, &typelong), exprconvert(r, &typelong));
+			return mkbinaryexpr(loc, TDIV, e, mkconstexpr(&typelong, l->type->base->size));
 		} else {
 			error(loc, "invalid operands to '-' operator");
 		}
diff --git a/tests/subtract-pointer.qbe b/tests/subtract-pointer.qbe
index 9e49d9c..0e7e9a3 100644
--- a/tests/subtract-pointer.qbe
+++ b/tests/subtract-pointer.qbe
@@ -8,10 +8,9 @@ function $f(l %.1, l %.3) {
 @body.2
 	%.5 =l loadl %.2
 	%.6 =l copy %.5
-	%.7 =l udiv %.6, 4
-	%.8 =l loadl %.4
-	%.9 =l copy %.8
-	%.10 =l udiv %.9, 4
-	%.11 =l sub %.7, %.10
+	%.7 =l loadl %.4
+	%.8 =l copy %.7
+	%.9 =l sub %.6, %.8
+	%.10 =l div %.9, 4
 	ret
 }
-- 
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