From e36dc296f50cb2d60d3b27ce62217762b9655313 Mon Sep 17 00:00:00 2001
From: Michael Forney <mforney@mforney.org>
Date: Wed, 29 Sep 2021 15:03:15 -0700
Subject: eval: Allow subtraction in address constants

---
 eval.c                                | 14 +++++++-------
 test/initializer-address-subtract.c   |  2 ++
 test/initializer-address-subtract.qbe |  2 ++
 3 files changed, 11 insertions(+), 7 deletions(-)
 create mode 100644 test/initializer-address-subtract.c
 create mode 100644 test/initializer-address-subtract.qbe

diff --git a/eval.c b/eval.c
index 958f517..0c68051 100644
--- a/eval.c
+++ b/eval.c
@@ -157,19 +157,19 @@ eval(struct expr *expr, enum evalkind kind)
 		case TADD:
 			if (r->kind == EXPRBINARY)
 				c = l, l = r, r = c;
+			/* fallthrough */
+		case TSUB:
 			if (r->kind != EXPRCONST)
 				break;
 			if (l->kind == EXPRCONST) {
 				binary(expr, expr->op, l, r);
-			} else if (l->kind == EXPRBINARY && l->type->kind == TYPEPOINTER && l->binary.r->kind == EXPRCONST) {
-				if (l->op == TADD || l->op == TSUB) {
-					/* (P ± C1) + C2  ->  P + (C2 ± C1) */
-					expr->binary.l = l->binary.l;
-					binary(expr->binary.r, l->op, r, l->binary.r);
-				}
+			} else if (l->kind == EXPRBINARY && l->type->kind == TYPEPOINTER && l->op == TADD && l->binary.r->kind == EXPRCONST) {
+				/* (P + C1) ± C2  ->  P + (C1 ± C2) */
+				binary(expr->binary.r, expr->op, l->binary.r, r);
+				expr->op = TADD;
+				expr->binary.l = l->binary.l;
 			}
 			break;
-		/* TODO: TSUB pointer handling */
 		case TLOR:
 			if (l->kind != EXPRCONST)
 				break;
diff --git a/test/initializer-address-subtract.c b/test/initializer-address-subtract.c
new file mode 100644
index 0000000..f273d33
--- /dev/null
+++ b/test/initializer-address-subtract.c
@@ -0,0 +1,2 @@
+int a[4];
+int *p = a + 2 - 1;
diff --git a/test/initializer-address-subtract.qbe b/test/initializer-address-subtract.qbe
new file mode 100644
index 0000000..e6aa8cd
--- /dev/null
+++ b/test/initializer-address-subtract.qbe
@@ -0,0 +1,2 @@
+export data $p = align 8 { l $a + 4, }
+export data $a = align 4 { z 16 }
-- 
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