Man, you should try Clang's `-Weverything` it's… well you know what, I'll try that on my stupidly high-quality cryptographic code:

$ make "CC=clang -std=c99" "CFLAGS=-O3 -Weverything"
clang -std=c99 -O3 -Weverything -I src -I src/optional -fPIC -c -o lib/monocypher.o src/monocypher.c
src/monocypher.c:262:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
u8 tmp[64];
^
src/monocypher.c:231:9: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
u32 pool[16];
^
src/monocypher.c:337:12: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
const u64 s0 = ctx->h[0] + (u64)s[0]; // s0 <= 1_fffffffe
^
In file included from src/monocypher.c:54:
src/monocypher.h:289:9: warning: padding size of 'crypto_poly1305_ctx' with 4 bytes to alignment boundary [-Wpadded]
typedef struct {
^
src/monocypher.c:410:9: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
size_t nb_blocks = message_size >> 4;
^
src/monocypher.c:437:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
u64 c = 5;
^
src/monocypher.c:498:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
u64 v0 = ctx->hash[0]; u64 v8 = iv[0];
^
src/monocypher.c:621:10: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
size_t nb_words = message_size >> 3;
^
src/monocypher.c:600:9: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
size_t nb_blocks = message_size >> 7;
^
src/monocypher.c:640:9: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
size_t hash_size = MIN(ctx->hash_size, 64);
^
src/monocypher.c:786:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
u8 hash_area[1024];
^
src/monocypher.c:874:10: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
u32 next_slice = ((slice + 1) % 4) * segment_size;
^
src/monocypher.c:801:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
int constant_time = config.algorithm != CRYPTO_ARGON2_D;
^
src/monocypher.c:1170:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
i32 q = (19 * t[9] + (((i32) 1) << 24)) >> 25;
^
src/monocypher.c:1337:5: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
u8 isodd = s[0] & 1;
^
src/monocypher.c:1349:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
int isdifferent = crypto_verify32(fs, gs);
^
src/monocypher.c:1433:7: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
i32 *quartic = t1;
^
src/monocypher.c:1500:5: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
fe x2, z2, x3, z3, t0, t1;
^
src/monocypher.c:1650:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
u64 carry = 1;
^
src/monocypher.c:1676:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
u32 B[8]; load32_le_buf(B, b, 8);
^
src/monocypher.c:1756:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
int is_square = invsqrt(h->X, h->X);
^
src/monocypher.c:1956:8: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
int lsb = v & (~v + 1); // smallest bit of v
^
src/monocypher.c:2015:7: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
int h_digit = slide_step(&h_slide, P_W_WIDTH, i, h);
^
src/monocypher.c:1994:12: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
ge_cached lutA[P_W_SIZE];
^
src/monocypher.c:2185:5: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
fe tmp_a, tmp_b; // temporaries for addition
^
src/monocypher.c:2540:5: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
fe t1, t2;
^
src/monocypher.c:2641:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
int is_square = invsqrt(t1, t1);
^
src/monocypher.c:2696:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
int is_square = invsqrt(t3, t3); // t3 = sqrt(-1 / non_square * u * (u+A))
^
src/monocypher.c:2775:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
u32 t[16] = {0};
^
src/monocypher.c:2815:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
u32 m_scl[8];
^
src/monocypher.c:2870:22: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
crypto_poly1305_ctx poly_ctx; // auto wiped...
^
src/monocypher.c:2925:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
int mismatch = crypto_verify16(mac, real_mac);
^
src/monocypher.c:2953:6: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
int mismatch = crypto_aead_read(&ctx, plain_text, mac, ad, ad_size,
^
33 warnings generated.
clang -std=c99 -O3 -Weverything -I src -I src/optional -fPIC -c -o lib/monocypher-ed25519.o src/optional/monocypher-ed25519.c
src/optional/monocypher-ed25519.c:165:8: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
u64 in = K[i16 + j] + ctx->input[j];
^
src/optional/monocypher-ed25519.c:158:9: warning: mixing declarations and code is incompatible with standards before C99 [-Wdeclaration-after-statement]
size_t i16 = 0;
^
2 warnings generated.
ar cr lib/libmonocypher.a lib/monocypher.o lib/monocypher-ed25519.o
clang -std=c99 -O3 -Weverything -shared -Wl,-soname,libmonocypher.so.4 -o lib/libmonocypher.so.4 lib/monocypher.o lib/monocypher-ed25519.o
ln -sf `basename lib/libmonocypher.so.4` lib/libmonocypher.so
doc/doc_gen.sh

---

Well…

Yeah.

That is cranking up warnings up to 11. Now this is C99 we're talking about, how about removing that obviously useless `-Wdeclaration-after-statement`?

$ make "CC=clang -std=c99" "CFLAGS=-O3 -Weverything -Wno-declaration-after-statement"
clang -std=c99 -O3 -Weverything -Wno-declaration-after-statement -I src -I src/optional -fPIC -c -o lib/monocypher.o src/monocypher.c
In file included from src/monocypher.c:54:
src/monocypher.h:289:9: warning: padding size of 'crypto_poly1305_ctx' with 4 bytes to alignment boundary [-Wpadded]
typedef struct {
^
1 warning generated.

Ah, this one's may be real. Let's see this struct:

typedef struct {
// Do not rely on the size or contents of this type,
// for they may change without notice.
uint8_t c[16]; // chunk of the message
size_t c_idx; // How many bytes are there in the chunk.
uint32_t r [4]; // constant multiplier (from the secret key)
uint32_t pad[4]; // random number added at the end (from the secret key)
uint32_t h [5]; // accumulated hash
} crypto_poly1305_ctx;

Ah, I see: we end by an odd number of 32-bit words, and in the 64-bit architecture I'm compiling this on, there will be 4 bytes of padding. But then I have two problems: first, who cares? Second, I also target 32-bit architectures on which there will not be any padding, and adding it manually would be wasteful.

There's the `#pragma` to silence the warning of course, but it's ugly, and I'm not sure it is strictly portable (unless the standard says compilers should ignore the `#pragma` if they don't understand it, but I confess haven't checked).