Extended Description

Starting with clang-9, I get a warning about the crypto/blake2b_generic.c implementation of the Linux kernel when building for armv7-m:

This does not happen with clang-8, or when targeting any other ARM version (v7-a, v5, ...) or any other architecture I tried. Also, using "-Os" optimization instead of "-O2" avoids the problem, using only the usual 480 bytes of stack.

Here is a reduced test case:

typedef unsigned char u8;
typedef unsigned int u32;
typedef unsigned long long u64;
static inline u64 ror64(u64 word, unsigned int shift)
{
return (word >> (shift & 63)) | (word << ((-shift) & 63));
}
struct blake2b_state { u64 h[8]; };
static const u64 blake2b_IV[8] =
{ 0x6a09e667f3bcc908ULL, 0xbb67ae8584caa73bULL, 0x3c6ef372fe94f82bULL, 0xa54ff53a5f1d36f1ULL,
0x510e527fade682d1ULL, 0x9b05688c2b3e6c1fULL, 0x1f83d9abfb41bd6bULL, 0x5be0cd19137e2179ULL };
static const u8 blake2b_sigma[12][16] =
{ { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 } };
#define g(r,i,a,b,c,d)
do {
a = a + b + m[blake2b_sigma[r][2*i+0]];
d = ror64(d ^ a, 32);
c = c + d;
b = ror64(b ^ c, 24);
d = ror64(d ^ a, 16);
b = ror64(b ^ c, 63);
} while (0)
#define ROUND(r)
do {
g(r,0,v[ 0],v[ 4],v[ 8],v[12]);
g(r,4,v[ 0],v[ 5],v[10],v[15]);
g(r,5,v[ 1],v[ 6],v[11],v[12]);
g(r,6,v[ 2],v[ 7],v[ 8],v[13]);
g(r,7,v[ 3],v[ 4],v[ 9],v[14]);
} while (0)
void blake2b_compress(struct blake2b_state *S,
const u8 block[])
{
u64 m[16];
u64 v[16];
unsigned long i;
for (i = 0; i < 16; ++i)
m[i] = (u64)(block + i * sizeof(m[i]));
for (i = 0; i < 8; ++i)
v[i] = S->h[i];
v[8] = blake2b_IV[0];
v[9] = blake2b_IV[1];
v[10] = blake2b_IV[2];
v[11] = blake2b_IV[3];
ROUND(0);
ROUND(1);
ROUND(2);
ROUND(3);
ROUND(4);
ROUND(5);
ROUND(6);
ROUND(7);
ROUND(8);
ROUND(9);
ROUND(10);
ROUND(11);
for (i = 0; i < 8; ++i)
S->h[i] = S->h[i] ^ v[i] ^ v[i + 8];
}

$ clang-11 -O2 -mthumb -Wframe-larger-than=100 --target=arm-linux -msoft-float -march=armv7-a test.c
warning: stack frame size of 1200 bytes in function 'blake2b_compress' [-Wframe-larger-than=]

See also https://godbolt.org/z/B5jCfH