Committed as obvious. The bitfield-struct t0 in gcc.dg/pr94600-1.c ..-4.c is assigned through a pointer that is a (volatile-and-pointer-)cast literal, so gcc doesn't need to be otherwise told that the address is aligned. But, variants pr94600-5.c ..-8.c are assigned through a "volatile t0 *", and rely on the *type* being naturally aligned, or that the machine has non-strict-alignment moves.
Unfortunately, systems exist (for some definitions of exist) where such structs aren't always naturally aligned, for example if it contains only (small) bitfields, even though the size is a naturally accessible size. Specifically, the mmix-knuth-mmixware port has only *byte* alignment for this struct. (If an int is added to the struct, alignment is promoted.) IOW, a prerequisite of the test is false: the struct doesn't have the same alignment as an integer of the same size. The effect is assignment in byte-size pieces, and the test fails. (For a non-volatile assignment, memcpy is called.) That's easily fixable by defining the type as having a specific alignment. This is also closer to the type in the original code, and also as the first variants aren't affected, no second thought or re-visit of pre-fixed compiler is needed. I don't plan to back-port this to gcc-10 branch however. I did sanity-check that the tests still pass on ppc64le-linux. gcc/testsuite: PR middle-end/94600 * gcc.dg/pr94600-5.c, gcc.dg/pr94600-6.c, gcc.dg/pr94600-7.c, gcc.dg/pr94600-8.c: Align t0 to 4-byte boundary. --- gcc/gcc/testsuite/gcc.dg/pr94600-5.c.orig Mon Jul 13 21:02:59 2020 +++ gcc/gcc/testsuite/gcc.dg/pr94600-5.c Sun Aug 9 05:03:32 2020 @@ -9,7 +9,7 @@ typedef struct { unsigned int f1 : 11; unsigned int f2 : 10; unsigned int f3 : 7; -} t0; +} t0 __attribute__((__aligned__(4))); static t0 a0[] = { { .f0 = 7, .f1 = 99, .f3 = 1, }, --- gcc/gcc/testsuite/gcc.dg/pr94600-6.c.orig Mon Jul 13 21:02:59 2020 +++ gcc/gcc/testsuite/gcc.dg/pr94600-6.c Sun Aug 9 05:05:36 2020 @@ -9,7 +9,7 @@ typedef struct { unsigned int f1 : 11; unsigned int f2 : 10; unsigned int f3 : 7; -} t0; +} t0 __attribute__((__aligned__(4))); void bar(volatile t0 *b) --- gcc/gcc/testsuite/gcc.dg/pr94600-7.c.orig Mon Jul 13 21:02:59 2020 +++ gcc/gcc/testsuite/gcc.dg/pr94600-7.c Sun Aug 9 05:05:47 2020 @@ -9,7 +9,7 @@ typedef struct { unsigned int f1 : 11; unsigned int f2 : 10; unsigned int f3 : 7; -} t0; +} t0 __attribute__((__aligned__(4))); static t0 a0[] = { { .f0 = 7, .f1 = 99, .f3 = 1, }, --- gcc/gcc/testsuite/gcc.dg/pr94600-8.c.orig Mon Jul 13 21:02:59 2020 +++ gcc/gcc/testsuite/gcc.dg/pr94600-8.c Sun Aug 9 05:05:54 2020 @@ -9,7 +9,7 @@ typedef struct { unsigned int f1 : 11; unsigned int f2 : 10; unsigned int f3 : 7; -} t0; +} t0 __attribute__((__aligned__(4))); void bar(volatile t0 *b)