Ihor Solodrai reported a case ([1]) where gcc reports an error but clang
ignores that error and proceeds to generate incorrect code. More
specifically, the problematic code looks like:
if r1 == 0xcafefeeddeadbeef goto <label>
Here, 0xcafefeeddeadbeef needs to be encoded in a 32-bit imm field
of the insns and the 32-bit imm allows sign extenstion to 64-bit imm.
Obviously, 0xcafefeeddeadbeef cannot encode properly.
The compilation failed for gcc with the following error:
Error: immediate out of range, shall fit in 32 bits
Given a 64-bit imm value, converting to the proper 32-bit imm value
must satisfy the following 64-bit patterns:
00000000 00000000 00000000 00000000 xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
11111111 11111111 11111111 11111111 1xxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
So if the top 32-bits is 0 or the top 33-bits is 0x1ffffffff, then the 64-bit imm
value can be truncated into proper 32-bit imm. Otherwise, a warning
message, the same as gcc, will be issued. If -Werror is enabled during
compilation, the warning will turn into an error.
[1] https://lore.kernel.org/bpf/70affb12-327b-4882-bd1d-afda8b8c6f56@linux.dev/
2f1e6eb6c3