https://gitlab.freedesktop.org/fontconfig/fontconfig/-/merge_requests/333

From 8db62f5a77fd53543dd58e12da7bd96082228893 Mon Sep 17 00:00:00 2001
From: Sam James <sam@gentoo.org>
Date: Tue, 8 Oct 2024 16:48:19 +0100
Subject: [PATCH] fontconfig: mark _FcPatternIter as may_alias

We had a report of GCC 14 with -O3 -flto causing wrong font
sizes with fontconfig (showing up in qalculate-gtk).

It turns out to be because _FcPatternIter and _FcPatternPrivateIter
are punned between which violates strict-aliasing rules, which
manifested in FcDefaultSubstitute getting a bogus value from
FcValueCanonicalize for size.

void* isn't allowed to alias anything -- you can pass addresses
around provided you cast back to the original type, but if you
access through the wrong type, you've violated aliasing rules.

Bug: https://bugs.gentoo.org/940923
Signed-off-by: Sam James <sam@gentoo.org>
--- a/fontconfig/fontconfig.h
+++ b/fontconfig/fontconfig.h
@@ -32,8 +32,10 @@
 
 #if defined(__GNUC__) && (__GNUC__ >= 4)
 #define FC_ATTRIBUTE_SENTINEL(x) __attribute__((__sentinel__(0)))
+#define FC_ATTRIBUTE_MAY_ALIAS __attribute__((may_alias))
 #else
 #define FC_ATTRIBUTE_SENTINEL(x)
+#define FC_ATTRIBUTE_MAY_ALIAS
 #endif
 
 #ifndef FcPublic
@@ -253,7 +255,7 @@ typedef enum _FcValueBinding {
 
 typedef struct _FcPattern   FcPattern;
 
-typedef struct _FcPatternIter {
+typedef struct FC_ATTRIBUTE_MAY_ALIAS _FcPatternIter {
     void *dummy1;
     void *dummy2;
 } FcPatternIter;
@@ -1160,6 +1162,7 @@ FcConfigParseAndLoadFromMemory (FcConfig       *config,
 _FCFUNCPROTOEND
 
 #undef FC_ATTRIBUTE_SENTINEL
+#undef FC_ATTRIBUTE_MAY_ALIAS
 
 
 #ifndef _FCINT_H_
-- 
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