rustc_codegen_cranelift
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Published
20 hours ago •
rust-lang
rust-lang
u128 comparison fails with opt-level 1
This little program is fine with opt 0, but fails with opt 1.
fn main() {
const X: u128 = 0x8000_0000_0000_0000;
assert_eq!((|| X)(), X);
}
$ rustc +nightly foo.rs -Zcodegen-backend=cranelift -Copt-level=1 && ./foo
thread 'main' panicked at foo.rs:3:5:
assertion `left == right` failed
left: 9223372036854775808
right: 9223372036854775808
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Note that this constant could fit in u64, but it looks like it's sign-extending that %r8 MSB into %rcx:
845c: e8 86 00 00 00 call 84e7 <_ZN3foo4main28_$u7b$$u7b$closure$u7d$$u7d$17h8cd7c7807c97b1efE>
8461: 48 8d 34 24 lea (%rsp),%rsi
8465: 48 89 06 mov %rax,(%rsi)
8468: 48 89 56 08 mov %rdx,0x8(%rsi)
846c: 48 8d 34 24 lea (%rsp),%rsi
8470: 48 8b 06 mov (%rsi),%rax
8473: 48 8b 56 08 mov 0x8(%rsi),%rdx
8477: 49 b8 00 00 00 00 00 movabs $0x8000000000000000,%r8
847e: 00 00 80
8481: 4c 89 c1 mov %r8,%rcx
8484: 48 c1 f9 3f sar $0x3f,%rcx
8488: 4c 39 c0 cmp %r8,%rax
848b: 41 0f 94 c0 sete %r8b
848f: 48 39 ca cmp %rcx,%rdx
8492: 41 0f 94 c1 sete %r9b
8496: 4d 21 c8 and %r9,%r8
8499: 49 f7 c0 01 00 00 00 test $0x1,%r8
84a0: 0f 84 09 00 00 00 je 84af <_ZN3foo4main17hb0af6480194903a8E+0x62>
84a6: 48 83 c4 50 add $0x50,%rsp
84aa: 48 89 ec mov %rbp,%rsp
84ad: 5d pop %rbp
84ae: c3 ret
The closure properly zeros the top half instead:
00000000000084e7 <_ZN3foo4main28_$u7b$$u7b$closure$u7d$$u7d$17h8cd7c7807c97b1efE>:
84e7: 55 push %rbp
84e8: 48 89 e5 mov %rsp,%rbp
84eb: 48 b8 00 00 00 00 00 movabs $0x8000000000000000,%rax
84f2: 00 00 80
84f5: 48 31 d2 xor %rdx,%rdx
84f8: 48 89 ec mov %rbp,%rsp
84fb: 5d pop %rbp
84fc: c3 ret
$ rustc +nightly -Vv
rustc 1.79.0-nightly (f9b161492 2024-04-19)
binary: rustc
commit-hash: f9b16149208c8a8a349c32813312716f6603eb6f
commit-date: 2024-04-19
host: x86_64-unknown-linux-gnu
release: 1.79.0-nightly
LLVM version: 18.1.4
-Zmir-opt-level=2 without -Copt-level=1 reproduces this too. Looks like
v3 = icmp_imm eq v2, 0x8000_0000_0000_0000
gets legalized to
v12 = iconst.i64 0x8000_0000_0000_0000
v13 = sextend.i128 v12 ; v12 = 0x8000_0000_0000_0000
v3 = icmp eq v2, v13
which is to be expected as immediates in clif ir are signed. The problem is that cranelift_frontend::switch::icmp_imm_u128 uses icmp_imm here despite the value to check against not fitting in an i64.
