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Identity Result match mapping should optimize to an identity function #38349
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Just leaving a note: This fix in LLVM will probably resolve the issue. |
copy propagation in MIR for function arguments (added in #38332) can cause this to optimize correctly, at least for the diff in unoptimized IR just points to the range metadata ending up somewhere it's not removed. |
So according to the issue @stjepang notes, it looks like this may have been merged into LLVM trunk on Mar 22 2017; in https://reviews.llvm.org/rL298540. It's possible we could backport this? I'm not sure, but this could be #40914. |
What I can see, this is still an issue in rustc 1.21.0-nightly (b75d1f0 2017-08-02) |
Fixed by #45380? Diff of unoptimized IRs (1.21 stable -> nightly): ; test::id_result
; Function Attrs: uwtable
-define void @_ZN4test9id_result17h0e83cb02e8136307E(%"core::result::Result<u64, i64>"* noalias nocapture sret dereferenceable(16), %"core::result::Result<u64, i64>"* noalias nocapture dereferenceable(16)) unnamed_addr #0 {
+define void @_ZN4test9id_result17ha0e8372a885d01bfE(%"core::result::Result<u64, i64>"* noalias nocapture sret dereferenceable(16), %"core::result::Result<u64, i64>"* noalias nocapture dereferenceable(16) %a) unnamed_addr #0 {
start:
- %a = alloca %"core::result::Result<u64, i64>"
- %2 = bitcast %"core::result::Result<u64, i64>"* %1 to i8*
- %3 = bitcast %"core::result::Result<u64, i64>"* %a to i8*
- call void @llvm.memcpy.p0i8.p0i8.i64(i8* %3, i8* %2, i64 16, i32 8, i1 false)
- %4 = getelementptr inbounds %"core::result::Result<u64, i64>", %"core::result::Result<u64, i64>"* %a, i32 0, i32 0
- %5 = load i64, i64* %4, !range !0
- switch i64 %5, label %bb2 [
+ %1 = getelementptr inbounds %"core::result::Result<u64, i64>", %"core::result::Result<u64, i64>"* %a, i32 0, i32 0
+ %2 = load i64, i64* %1, !range !0
+ switch i64 %2, label %bb2 [
i64 0, label %bb1
]
(snip) Optimized IR on nightly (1.23.0-nightly (45caff8 2017-11-11)): ; test::id_result
; Function Attrs: norecurse nounwind uwtable
define void @_ZN4test9id_result17ha0e8372a885d01bfE(%"core::result::Result<u64, i64>"* noalias nocapture sret dereferenceable(16), %"core::result::Result<u64, i64>"* noalias nocapture readonly dereferenceable(16) %a) unnamed_addr #0 {
start:
%1 = bitcast %"core::result::Result<u64, i64>"* %a to <2 x i64>*
%2 = load <2 x i64>, <2 x i64>* %1, align 8
%3 = bitcast %"core::result::Result<u64, i64>"* %0 to <2 x i64>*
store <2 x i64> %2, <2 x i64>* %3, align 8
ret void
} |
Yep, that's fixed in the original code this bug was filed for. The issue persists if we tweak it just a little (32-bit instead of 64-bit payload): #![crate_type="lib"]
type T = i32;
#[no_mangle]
pub fn id_result(a: Result<T, T>) -> Result<T, T> {
match a {
Ok(x) => Ok(x),
Err(y) => Err(y),
}
} |
@Mark-Simulacrum Is that patch only for |
I don't know. |
Hmm. Interesting, the second case now becomes Rust MIR: fn id_result(_1: Result<i32, i32>) -> Result<i32, i32> {
debug a => _1; // in scope 0 at src/lib.rs:6:18: 6:19
let mut _0: std::result::Result<i32, i32>; // return place in scope 0 at src/lib.rs:6:38: 6:50
let mut _2: isize; // in scope 0 at src/lib.rs:8:9: 8:14
let _3: i32; // in scope 0 at src/lib.rs:8:12: 8:13
let mut _4: i32; // in scope 0 at src/lib.rs:8:21: 8:22
let _5: i32; // in scope 0 at src/lib.rs:9:13: 9:14
let mut _6: i32; // in scope 0 at src/lib.rs:9:23: 9:24
scope 1 {
debug x => _3; // in scope 1 at src/lib.rs:8:12: 8:13
}
scope 2 {
debug y => _5; // in scope 2 at src/lib.rs:9:13: 9:14
}
bb0: {
_2 = discriminant(_1); // scope 0 at src/lib.rs:7:11: 7:12
switchInt(move _2) -> [0_isize: bb3, 1_isize: bb1, otherwise: bb2]; // scope 0 at src/lib.rs:7:5: 7:12
}
bb1: {
StorageLive(_5); // scope 0 at src/lib.rs:9:13: 9:14
_5 = ((_1 as Err).0: i32); // scope 0 at src/lib.rs:9:13: 9:14
StorageLive(_6); // scope 2 at src/lib.rs:9:23: 9:24
_6 = _5; // scope 2 at src/lib.rs:9:23: 9:24
Deinit(_0); // scope 2 at src/lib.rs:9:19: 9:25
((_0 as Err).0: i32) = move _6; // scope 2 at src/lib.rs:9:19: 9:25
discriminant(_0) = 1; // scope 2 at src/lib.rs:9:19: 9:25
StorageDead(_6); // scope 2 at src/lib.rs:9:24: 9:25
StorageDead(_5); // scope 0 at src/lib.rs:9:24: 9:25
goto -> bb4; // scope 0 at src/lib.rs:9:24: 9:25
}
bb2: {
unreachable; // scope 0 at src/lib.rs:7:11: 7:12
}
bb3: {
StorageLive(_3); // scope 0 at src/lib.rs:8:12: 8:13
_3 = ((_1 as Ok).0: i32); // scope 0 at src/lib.rs:8:12: 8:13
StorageLive(_4); // scope 1 at src/lib.rs:8:21: 8:22
_4 = _3; // scope 1 at src/lib.rs:8:21: 8:22
Deinit(_0); // scope 1 at src/lib.rs:8:18: 8:23
((_0 as Ok).0: i32) = move _4; // scope 1 at src/lib.rs:8:18: 8:23
discriminant(_0) = 0; // scope 1 at src/lib.rs:8:18: 8:23
StorageDead(_4); // scope 1 at src/lib.rs:8:22: 8:23
StorageDead(_3); // scope 0 at src/lib.rs:8:22: 8:23
goto -> bb4; // scope 0 at src/lib.rs:8:22: 8:23
}
bb4: {
return; // scope 0 at src/lib.rs:11:2: 11:2
}
} LLVMIR: ; ModuleID = 'playground.a04b7f1e-cgu.0'
source_filename = "playground.a04b7f1e-cgu.0"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
; Function Attrs: mustprogress nofree norecurse nosync nounwind nonlazybind readnone uwtable willreturn
define { i32, i32 } @id_result(i32 noundef %0, i32 %1) unnamed_addr #0 {
start:
%switch = icmp ne i32 %0, 0
%. = zext i1 %switch to i32
%2 = insertvalue { i32, i32 } undef, i32 %., 0
%3 = insertvalue { i32, i32 } %2, i32 %1, 1
ret { i32, i32 } %3
}
attributes #0 = { mustprogress nofree norecurse nosync nounwind nonlazybind readnone uwtable willreturn "probe-stack"="__rust_probestack" "target-cpu"="x86-64" }
!llvm.module.flags = !{!0, !1}
!0 = !{i32 7, !"PIC Level", i32 2}
!1 = !{i32 2, !"RtLibUseGOT", i32 1} x86 assembly: id_result: # @id_result
# %bb.0:
movl %esi, %edx
xorl %eax, %eax
testl %edi, %edi
setne %al
retq
# -- End function The MIR emitted is identical, however, so it has to be regarding how we are handling Results of particular sizes or shapes during lowering of them, somewhere in cg_ssa or cg_llvm. |
There is no |
This was isolated by @stjepang in #37939
This program: (playground)
Should optimize to something that just copies the input to the output, with no conditionals.
Output on
rustc 1.15.0-nightly (daf8c1dfc 2016-12-05)
using-Copt-level=3 --emit=llvm-ir
is this:The text was updated successfully, but these errors were encountered: