From ebdae640721cd7be00db01b6e5d0793b46926f3b Mon Sep 17 00:00:00 2001
From: Michael Forney <mforney@mforney.org>
Date: Sun, 11 Apr 2021 17:35:43 -0700
Subject: decl: Allow _Alignas(0)

C11 6.7.5p6 says "An alignment specification of zero has no effect".
---
 decl.c                | 5 +++--
 test/alignas-0.c      | 3 +++
 test/alignas-0.qbe    | 3 +++
 test/alignas-type.c   | 1 +
 test/alignas-type.qbe | 1 +
 test/alignas.c        | 2 +-
 test/alignas.qbe      | 2 +-
 7 files changed, 13 insertions(+), 4 deletions(-)
 create mode 100644 test/alignas-0.c
 create mode 100644 test/alignas-0.qbe
 create mode 100644 test/alignas-type.c
 create mode 100644 test/alignas-type.qbe

diff --git a/decl.c b/decl.c
index 94e4b15..0c3b037 100644
--- a/decl.c
+++ b/decl.c
@@ -379,9 +379,10 @@ declspecs(struct scope *s, enum storageclass *sc, enum funcspec *fs, int *align)
 				*align = other->align;
 			} else {
 				i = intconstexpr(s, false);
-				if (!i || i & (i - 1) || i > 16)
+				if (i & (i - 1) || i > 16)
 					error(&tok.loc, "invalid alignment: %d", i);
-				*align = (int)i;
+				if (i)
+					*align = (int)i;
 			}
 			expect(TRPAREN, "to close '_Alignas' specifier");
 			break;
diff --git a/test/alignas-0.c b/test/alignas-0.c
new file mode 100644
index 0000000..402e7ee
--- /dev/null
+++ b/test/alignas-0.c
@@ -0,0 +1,3 @@
+_Alignas(0) int x;
+_Alignas(8) _Alignas(0) int y;
+_Alignas(0) _Alignas(16) int z;
diff --git a/test/alignas-0.qbe b/test/alignas-0.qbe
new file mode 100644
index 0000000..849fb58
--- /dev/null
+++ b/test/alignas-0.qbe
@@ -0,0 +1,3 @@
+export data $x = align 4 { z 4 }
+export data $y = align 8 { z 4 }
+export data $z = align 16 { z 4 }
diff --git a/test/alignas-type.c b/test/alignas-type.c
new file mode 100644
index 0000000..394045d
--- /dev/null
+++ b/test/alignas-type.c
@@ -0,0 +1 @@
+_Alignas(int) char x[4];
diff --git a/test/alignas-type.qbe b/test/alignas-type.qbe
new file mode 100644
index 0000000..7cf0e40
--- /dev/null
+++ b/test/alignas-type.qbe
@@ -0,0 +1 @@
+export data $x = align 4 { z 4 }
diff --git a/test/alignas.c b/test/alignas.c
index 394045d..b714d37 100644
--- a/test/alignas.c
+++ b/test/alignas.c
@@ -1 +1 @@
-_Alignas(int) char x[4];
+_Alignas(8) int x;
diff --git a/test/alignas.qbe b/test/alignas.qbe
index 7cf0e40..6dd5505 100644
--- a/test/alignas.qbe
+++ b/test/alignas.qbe
@@ -1 +1 @@
-export data $x = align 4 { z 4 }
+export data $x = align 8 { z 4 }
-- 
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