Improve nop-match simplification #68481
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oli-obk
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Jan 24, 2020
| /// or | ||
| /// | ||
| /// ```rust | ||
| /// StorageLive(_LOCAL_TMP); |
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Can you elaborate on why we should be doing this optimization in debug mode (and why we shouldn't instead run the storage and temporary elimination)? Will this make debug mode compile faster in all cases?
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Sorry for the late reply!
AFAIK mir-opt-level is always 1 unless explicitly specified, regardless of release or debug build. mir-opt-level is independent from optimization level, and cargo does not set it. (-Zmir-optlevel is an unstable flag and setting it to 2 or larger is prone to ICE/miscompile anyway.)
You can confirm this by building MIR of this playground in release / debug build. Temporaries are not eliminated even in release build, and SimplifyArmIdentity is not working.
That said, the purpose of this PR is to improve the codegen of identical match in relase build.
diff of cargo rustc --release -- --emit=llvm-ir, using the same code as the playground above, before/after this PR
--- old.ll 2020-01-27 07:04:08.034979599 +0900
+++ new.ll 2020-01-27 07:04:21.682132255 +0900
@@ -1,5 +1,5 @@
-; ModuleID = 'foo.1lzogcxs-cgu.0'
-source_filename = "foo.1lzogcxs-cgu.0"
+; ModuleID = 'foo.2my2volq-cgu.0'
+source_filename = "foo.2my2volq-cgu.0"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
@@ -9,80 +9,35 @@
%"unwind::libunwind::_Unwind_Context" = type { [0 x i8] }
; foo::try_identity
-; Function Attrs: nofree norecurse nounwind nonlazybind uwtable
-define void @_ZN3foo12try_identity17hf7a3f301f9e3a548E(%"core::result::Result<u64, i32>"* noalias nocapture sret dereferenceable(16), %"core::result::Result<u64, i32>"* noalias nocapture readonly dereferenceable(16) %x) unnamed_addr #0 {
+; Function Attrs: nounwind nonlazybind uwtable
+define void @_ZN3foo12try_identity17h273ad7d1ef012f0eE(%"core::result::Result<u64, i32>"* noalias nocapture sret dereferenceable(16), %"core::result::Result<u64, i32>"* noalias nocapture readonly dereferenceable(16) %x) unnamed_addr #0 {
start:
- %1 = getelementptr inbounds %"core::result::Result<u64, i32>", %"core::result::Result<u64, i32>"* %x, i64 0, i32 0, i64 0
- %2 = load i32, i32* %1, align 8, !range !2
- %switch = icmp eq i32 %2, 1
- br i1 %switch, label %bb3, label %bb2
-
-bb2: ; preds = %start
- %3 = getelementptr inbounds %"core::result::Result<u64, i32>", %"core::result::Result<u64, i32>"* %x, i64 0, i32 2, i64 1
- %4 = bitcast i32* %3 to i64*
- %x2 = load i64, i64* %4, align 8
- %5 = getelementptr inbounds %"core::result::Result<u64, i32>", %"core::result::Result<u64, i32>"* %0, i64 0, i32 2, i64 1
- %6 = bitcast i32* %5 to i64*
- store i64 %x2, i64* %6, align 8
- br label %bb4
-
-bb3: ; preds = %start
- %7 = getelementptr inbounds %"core::result::Result<u64, i32>", %"core::result::Result<u64, i32>"* %x, i64 0, i32 2, i64 0
- %x1 = load i32, i32* %7, align 4
- %8 = getelementptr inbounds %"core::result::Result<u64, i32>", %"core::result::Result<u64, i32>"* %0, i64 0, i32 2, i64 0
- store i32 %x1, i32* %8, align 4
- br label %bb4
-
-bb4: ; preds = %bb2, %bb3
- %.sink = phi i32 [ 0, %bb2 ], [ 1, %bb3 ]
- %9 = getelementptr inbounds %"core::result::Result<u64, i32>", %"core::result::Result<u64, i32>"* %0, i64 0, i32 0, i64 0
- store i32 %.sink, i32* %9, align 8
+ %1 = bitcast %"core::result::Result<u64, i32>"* %0 to i8*
+ %2 = bitcast %"core::result::Result<u64, i32>"* %x to i8*
+ tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* nonnull align 8 %1, i8* nonnull align 8 %2, i64 16, i1 false)
ret void
}
; foo::try_identity_drop
; Function Attrs: nounwind nonlazybind uwtable
-define void @_ZN3foo17try_identity_drop17h5d9ea5a8e7421c58E(%"core::result::Result<alloc::string::String, i32>"* noalias nocapture sret dereferenceable(32), %"core::result::Result<alloc::string::String, i32>"* noalias nocapture readonly dereferenceable(32) %x) unnamed_addr #1 personality i32 (i32, i32, i64, %"unwind::libunwind::_Unwind_Exception"*, %"unwind::libunwind::_Unwind_Context"*)* @rust_eh_personality {
+define void @_ZN3foo17try_identity_drop17h7d0574702d5fadd5E(%"core::result::Result<alloc::string::String, i32>"* noalias nocapture sret dereferenceable(32), %"core::result::Result<alloc::string::String, i32>"* noalias nocapture readonly dereferenceable(32) %x) unnamed_addr #0 personality i32 (i32, i32, i64, %"unwind::libunwind::_Unwind_Exception"*, %"unwind::libunwind::_Unwind_Context"*)* @rust_eh_personality {
start:
- %1 = getelementptr inbounds %"core::result::Result<alloc::string::String, i32>", %"core::result::Result<alloc::string::String, i32>"* %x, i64 0, i32 0, i64 0
- %2 = load i32, i32* %1, align 8, !range !2
- %switch = icmp eq i32 %2, 1
- br i1 %switch, label %bb8, label %bb5
-
-bb4: ; preds = %bb5, %bb8
- %.sink = phi i32 [ 0, %bb5 ], [ 1, %bb8 ]
- %3 = getelementptr inbounds %"core::result::Result<alloc::string::String, i32>", %"core::result::Result<alloc::string::String, i32>"* %0, i64 0, i32 0, i64 0
- store i32 %.sink, i32* %3, align 8
+ %1 = bitcast %"core::result::Result<alloc::string::String, i32>"* %0 to i8*
+ %2 = bitcast %"core::result::Result<alloc::string::String, i32>"* %x to i8*
+ tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* nonnull align 8 %1, i8* nonnull align 8 %2, i64 32, i1 false)
ret void
-
-bb5: ; preds = %start
- %x1.sroa.0.0..sroa_idx3 = getelementptr inbounds %"core::result::Result<alloc::string::String, i32>", %"core::result::Result<alloc::string::String, i32>"* %x, i64 0, i32 2, i64 1
- %x1.sroa.0.0..sroa_cast = bitcast i32* %x1.sroa.0.0..sroa_idx3 to i8*
- %_4.sroa.0.0..sroa_idx10 = getelementptr inbounds %"core::result::Result<alloc::string::String, i32>", %"core::result::Result<alloc::string::String, i32>"* %0, i64 0, i32 2, i64 1
- %_4.sroa.0.0..sroa_cast = bitcast i32* %_4.sroa.0.0..sroa_idx10 to i8*
- call void @llvm.memcpy.p0i8.p0i8.i64(i8* nonnull align 8 %_4.sroa.0.0..sroa_cast, i8* nonnull align 8 %x1.sroa.0.0..sroa_cast, i64 24, i1 false)
- br label %bb4
-
-bb8: ; preds = %start
- %4 = getelementptr inbounds %"core::result::Result<alloc::string::String, i32>", %"core::result::Result<alloc::string::String, i32>"* %x, i64 0, i32 2, i64 0
- %x2 = load i32, i32* %4, align 4
- %5 = getelementptr inbounds %"core::result::Result<alloc::string::String, i32>", %"core::result::Result<alloc::string::String, i32>"* %0, i64 0, i32 2, i64 0
- store i32 %x2, i32* %5, align 4
- br label %bb4
}
; Function Attrs: nounwind nonlazybind uwtable
-declare i32 @rust_eh_personality(i32, i32, i64, %"unwind::libunwind::_Unwind_Exception"*, %"unwind::libunwind::_Unwind_Context"*) unnamed_addr #1
+declare i32 @rust_eh_personality(i32, i32, i64, %"unwind::libunwind::_Unwind_Exception"*, %"unwind::libunwind::_Unwind_Context"*) unnamed_addr #0
; Function Attrs: argmemonly nounwind
-declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture writeonly, i8* nocapture readonly, i64, i1 immarg) #2
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture writeonly, i8* nocapture readonly, i64, i1 immarg) #1
-attributes #0 = { nofree norecurse nounwind nonlazybind uwtable "probe-stack"="__rust_probestack" "target-cpu"="x86-64" }
-attributes #1 = { nounwind nonlazybind uwtable "probe-stack"="__rust_probestack" "target-cpu"="x86-64" }
-attributes #2 = { argmemonly nounwind }
+attributes #0 = { nounwind nonlazybind uwtable "probe-stack"="__rust_probestack" "target-cpu"="x86-64" }
+attributes #1 = { argmemonly nounwind }
!llvm.module.flags = !{!0, !1}
!0 = !{i32 7, !"PIC Level", i32 2}
!1 = !{i32 2, !"RtLibUseGOT", i32 1}
-!2 = !{i32 0, i32 2}There was a problem hiding this comment.
Regarding the compiler performance: I locally run rustc-perf, and it looks perf-neutral. The 2nd point in #66234 (comment) is still required for these optimizations to be useful in real cases.
Improved SimplifyArmIdentity
Improved SimplifyArmIdentity + SinkCommonCodeFromPredecessors
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So there's two things I'm worried about here:
-
we're slowly adding more optimizations to debug mode, which by it's very definition should be easy to debug with a debugger. Such optimizations can easily end up messing up the source. This one doesn't do it yet, but still.
-
You essentially had to implement an optimization twice, once for mir-opt-level 1 and once for mir-opt-level 2. I feel like that needlessly complicates our optimizations.
Instead of implementing this optimization twice, how about moving the ICEing optimizations to mir-opt-level 3 and making mir-opt-level 2 the standard for --release?
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we're slowly adding more optimizations to debug mode, which by it's very definition should be easy to debug with a debugger. Such optimizations can easily end up messing up the source.
I realized this PR is already debuginfo-breaking. If the debugger was stepping on match, it would point to whichever arm the optimizer reduced all other arms into.
Instead of implementing this optimization twice, how about moving the ICEing optimizations to mir-opt-level 3 and making mir-opt-level 2 the standard for --release?
Your plan sounds right. Efforts should be concentrated on fixing/improving advanced MIR optimizations rather than adding ad-hoc optimization passes.
I'll close this PR. Thank you for your time, @oli-obk!
| @@ -169,19 +194,23 @@ fn match_arm(stmts: &mut [Statement<'_>], local_decls: &mut IndexVec<Local, Loca | |||
| return; | |||
| } | |||
|
|
|||
| if vf_1 != vf_0 // The field-and-variant information match up. | |||
| if drop_flag == Some(local_0) | |||
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I don't get what these checks do. The type of local_0 is never bool as far as I can tell, so how can local_0 and drop_flag refer to the same local?
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Yes, this one is definitely unnecessary. I added it by mistake - sorry for confusion.
| let mut matched_stmts = 0; | ||
| let basic_blocks = body.basic_blocks_mut(); | ||
|
|
||
| loop { |
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So... this loop goes backwards in all preds statements and gives you the number of statements that are the same? Basically extracting a common postfix of statements from all previous basic blocks?
| || drop_flag == Some(local_tmp_2) | ||
| || local_decls[local_tmp_2].is_user_variable() |
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Since I still don't understand what the above code is doing, I can't tell if removing the == Some(local_0) is ok
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== Some(local_0) surely didn't make sense as you pointed out. Regarding other checks... I'm not sure. I added checks to make sure the code being matched is exactly what I intended, but probably they can all be removed.
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SimplifyArmIdentitynot working in presence of temporaries and storage markes, which are not eliminated inmir-opt-level=1. This does not fix the issue of?not being eliminated inmir-opt-level < 2.SinkCommonCodeFromPredecessorspass. This allows optimizing identical matches on types that have destructors.This code ...
... gives you MIR with drop flag manipulation:
In this case
SimplifyBranchSamedoes not work becausebb2andbb3is still unequal afterSimplifyArmIdentitypass. It seems that LLVM is able to optimize thematchintomemcpyfrom this state, though.SinkCommonCodeFromPredecessorsis able to sink the common statement,_0 = move _1, tobb8.I stole the idea from LLVM, but this PR implements only (1) in the comment there.
#66234