treewide: Disable gcc strict aliasing rules as needed, drop workarounds

Recently, commit 4ddbcb9c0c ("tcp: Disable optimisations
for tcp_hash()") worked around yet another issue we hit with gcc 12
and '-flto -O2': some stores affecting the input data to siphash_20b()
were omitted altogether, and tcp_hash() wouldn't get the correct hash
for incoming connections.

Digging further into this revealed that, at least according to gcc's
interpretation of C99 aliasing rules, passing pointers to functions
with different types compared to the effective type of the object
(for example, a uint8_t pointer to an anonymous struct, as it happens
in tcp_hash()), doesn't guarantee that stores are not reordered
across the function call.

This means that, in general, our checksum and hash functions might
not see parts of input data that was intended to be provided by
callers.

Not even switching from uint8_t to character types, which should be
appropriate here, according to C99 (ISO/IEC 9899, TC3, draft N1256),
section 6.5, "Expressions", paragraph 7:

  An object shall have its stored value accessed only by an lvalue
  expression that has one of the following types:

  [...]

  — a character type.

does the trick. I guess this is also subject to interpretation:
casting passed pointers to character types, and then using those as
different types, might still violate (dubious) aliasing rules.

Disable gcc strict aliasing rules for potentially affected functions,
which, in turn, disables gcc's Type-Based Alias Analysis (TBAA)
optimisations based on those function arguments.

Drop the existing workarounds. Also the (seemingly?) bogus
'maybe-uninitialized' warning on the tcp_tap_handler() > tcp_hash() >
siphash_20b() path goes away with -fno-strict-aliasing on
siphash_20b().

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

checksum.c

@@ -69,6 +69,8 @@
  *
  * Return: 32-bit sum of 16-bit words
 */
+/* NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) */
+__attribute__((optimize("-fno-strict-aliasing")))	/* See siphash_8b() */
 uint32_t sum_16b(const void *buf, size_t len)
 {
 	const uint16_t *p = buf;
@@ -107,9 +109,8 @@ uint16_t csum_fold(uint32_t sum)
  *
  * Return: 16-bit IPv4-style checksum
  */
-#if CSUM_UNALIGNED_NO_IPA
-__attribute__((__noipa__))	/* See comment in Makefile */
-#endif
+/* NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) */
+__attribute__((optimize("-fno-strict-aliasing")))	/* See siphash_8b() */
 uint16_t csum_unaligned(const void *buf, size_t len, uint32_t init)
 {
 	return (uint16_t)~csum_fold(sum_16b(buf, len) + init);
@@ -245,6 +246,8 @@ void csum_icmp6(struct icmp6hdr *icmp6hr,
  * - sum_a/sum_b unpacking is interleaved and not sequential to reduce stalls
  * - coding style adaptation
  */
+/* NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) */
+__attribute__((optimize("-fno-strict-aliasing")))	/* See siphash_8b() */
 static uint32_t csum_avx2(const void *buf, size_t len, uint32_t init)
 {
 	__m256i a, b, sum256, sum_a_hi, sum_a_lo, sum_b_hi, sum_b_lo, c, d;
@@ -391,6 +394,8 @@ less_than_128_bytes:
  *
  * Return: 16-bit folded, complemented checksum sum
  */
+/* NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) */
+__attribute__((optimize("-fno-strict-aliasing")))	/* See siphash_8b() */
 uint16_t csum(const void *buf, size_t len, uint32_t init)
 {
 	return (uint16_t)~csum_fold(csum_avx2(buf, len, init));
@@ -406,6 +411,8 @@ uint16_t csum(const void *buf, size_t len, uint32_t init)
  *
  * Return: 16-bit folded, complemented checksum
  */
+/* NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) */
+__attribute__((optimize("-fno-strict-aliasing")))	/* See siphash_8b() */
 uint16_t csum(const void *buf, size_t len, uint32_t init)
 {
 	return csum_unaligned(buf, len, init);