https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102627
--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The assembly difference r11-8007 to r11-8008 is:
--- pr102627.s 2021-10-06 06:32:46.000000000 -0400
+++ pr102627.s 2021-10-06 06:33:00.000000000 -0400
@@ -77,10 +77,10 @@ main:
movq %rdx, %rcx
movq %rax, %rdx
movq e(%rip), %rax
- movq %rcx, 8(%rsp)
+ movl %ecx, 12(%rsp)
movzbl f(%rip), %ecx
salq %cl, %rax
- movq 8(%rsp), %rcx
+ movl 12(%rsp), %ecx
movq %rax, %rsi
movl $0, %edi
call w
I believe y returns the 128-bit struct g return value in %rdx:%rax pair, right
before the above instructions, and the above change means that instead of
spilling the whole 64-bits of %rcx that holds at that point u.j and u.k members
(u.k in the upper 32 bits of %rcx) it spills just 32-bits of %ecx and fills it
back in, effectively setting u.k to 0. The w call then takes %rdi, %rsi
arguments it doesn't use and the TImode in %rcx:%rdx pair, but with the high 32
bits of the TImode value lost. The reason for the spill is clear, the shift
instruction needs that register...
