From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alexander Korotkov <aekorotkov@gmail.com>
cc: Heikki Linnakangas <hlinnaka@iki.fi>,
        Alexander Korotkov <akorotkov@postgresql.org>,
        pgsql-hackers@lists.postgresql.org
Subject: Re: Detecting pointer misalignment (was Re: pgsql: Implementation of
 subscripting for jsonb)
In-reply-to: <477102.1612747200@sss.pgh.pa.us>
References: <E1l6Jjn-0000kp-75@gemulon.postgresql.org>
 <37bb56a0-391d-759d-771b-9c1d8c15a84b@iki.fi>
 <3293925.1612163110@sss.pgh.pa.us>
 <CAPpHfdsne3=T=fMNU45PtxdhSL_J2PjLTeS8rwKnJzUR4YNd4w@mail.gmail.com>
 <CAPpHfdu8Jh0tySa50Zhkyg6gMvi2pQFdHAMiF+UkZNqsDLwTQw@mail.gmail.com>
 <475514.1612745257@sss.pgh.pa.us> <477102.1612747200@sss.pgh.pa.us>
Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Sun, 07 Feb 2021 20:20:00 -0500"
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Date: Mon, 08 Feb 2021 11:49:41 -0500
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I wrote:
> After digging in gcc's release history, it seems they invented
> "-fsanitize=alignment" in GCC 5, so we can make this work for gcc
> by writing
> #if __GNUC__ >= 5
> (the likely() macro already uses a similar approach).  Can't say
> if that's close enough for clang too.

Ugh, no it isn't: even pretty recent clang releases only define
__GNUC__ as 4.  It looks like we need a separate test on clang's
version.  I looked at their version history and sanitizers seem
to have come in around clang 7, so I propose the attached (where
I worked a bit harder on the comment, too).

			regards, tom lane


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diff --git a/src/include/c.h b/src/include/c.h
index ae978830da..a86342093e 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -132,6 +132,18 @@
 #define pg_nodiscard
 #endif
 =

+/*
+ * Place this macro before functions that should be allowed to make misal=
igned
+ * accesses.  Think twice before using it on non-x86-specific code!
+ * Testing can be done with "-fsanitize=3Dalignment -fsanitize-trap=3Dali=
gnment"
+ * on clang, or "-fsanitize=3Dalignment -fno-sanitize-recover=3Dalignment=
" on gcc.
+ */
+#if __clang_major__ >=3D 7 || __GNUC__ >=3D 5
+#define pg_attribute_no_sanitize_alignment() __attribute__((no_sanitize("=
alignment")))
+#else
+#define pg_attribute_no_sanitize_alignment()
+#endif
+
 /*
  * Append PG_USED_FOR_ASSERTS_ONLY to definitions of variables that are o=
nly
  * used in assert-enabled builds, to avoid compiler warnings about unused
diff --git a/src/port/pg_crc32c_sse42.c b/src/port/pg_crc32c_sse42.c
index 3b94a7388a..10fc01e1f0 100644
--- a/src/port/pg_crc32c_sse42.c
+++ b/src/port/pg_crc32c_sse42.c
@@ -18,6 +18,7 @@
 =

 #include "port/pg_crc32c.h"
 =

+pg_attribute_no_sanitize_alignment()
 pg_crc32c
 pg_comp_crc32c_sse42(pg_crc32c crc, const void *data, size_t len)
 {

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