e8d5db206c
In https://reviews.llvm.org/D64235 a new algorithm has been introduced for updating the branch weights of latch blocks and their copies. It increases the probability of going to the exit block for each next peel iteration, calculating weights by (F - I * E, E), where: - F is a weight of the edge from latch to header. - E is a weight of the edge from latch to exit. - I is a number of peeling iteration. E.g: Let's say the latch branch weights are (100,300) and the estimated trip count is 4. If we peel off all 4 iterations the weights of the copied branches will be: 0: (100,300) 1: (100,200) 2: (100,100) 3: (100,1) https://godbolt.org/z/93KnoEsT6 So we make the original loop almost unreachable from the 3rd peeled copy according to the profile data. But that's only true if the profiling data is accurate. Underestimated trip count can lead to a performance issues with the register allocator, which may decide to spill intervals inside the loop assuming it's unreachable. Since we don't know how accurate the profiling data is, it seems better to set neutral 1/1 weights on the last peeled latch branch. After this change, the weights in the example above will look like this: 0: (100,300) 1: (100,200) 2: (100,100) 3: (100,100) Co-authored-by: Aleksandr Popov <apopov@azul.com>
90 lines
3.6 KiB
LLVM
90 lines
3.6 KiB
LLVM
; REQUIRES: asserts
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; RUN: opt < %s -S -debug-only=loop-unroll -passes=loop-unroll -unroll-runtime 2>&1 | FileCheck %s
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; RUN: opt < %s -S -debug-only=loop-unroll -passes='require<profile-summary>,function(require<opt-remark-emit>,loop-unroll)' 2>&1 | FileCheck %s
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; RUN: opt < %s -S -debug-only=loop-unroll -passes='require<profile-summary>,function(require<opt-remark-emit>,loop-unroll<no-profile-peeling>)' 2>&1 | FileCheck %s --check-prefixes=CHECK-NO-PEEL
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; Make sure we use the profile information correctly to peel-off 3 iterations
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; from the loop, and update the branch weights for the peeled loop properly.
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; All side exits to deopt does not change weigths.
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; CHECK: Loop Unroll: F[basic]
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; CHECK: PEELING loop %for.body with iteration count 4!
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; CHECK-NO-PEEL-NOT: PEELING loop %for.body
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; CHECK-LABEL: @basic
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; CHECK: br i1 %c, label %{{.*}}, label %side_exit, !prof !15
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; CHECK: br i1 %{{.*}}, label %[[NEXT0:.*]], label %for.cond.for.end_crit_edge, !prof !16
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; CHECK: [[NEXT0]]:
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; CHECK: br i1 %c, label %{{.*}}, label %side_exit, !prof !15
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; CHECK: br i1 %{{.*}}, label %[[NEXT1:.*]], label %for.cond.for.end_crit_edge, !prof !17
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; CHECK: [[NEXT1]]:
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; CHECK: br i1 %c, label %{{.*}}, label %side_exit, !prof !15
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; CHECK: br i1 %{{.*}}, label %[[NEXT2:.*]], label %for.cond.for.end_crit_edge, !prof !18
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; CHECK: [[NEXT2]]:
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; CHECK: br i1 %c, label %{{.*}}, label %side_exit.loopexit, !prof !15
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; CHECK: br i1 %{{.*}}, label %for.body, label %{{.*}}, !prof !18
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define i32 @basic(ptr %p, i32 %k, i1 %c) #0 !prof !15 {
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entry:
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%cmp3 = icmp slt i32 0, %k
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br i1 %cmp3, label %for.body.lr.ph, label %for.end
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for.body.lr.ph: ; preds = %entry
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br label %for.body
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for.body: ; preds = %for.body.lr.ph, %for.body
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%i.05 = phi i32 [ 0, %for.body.lr.ph ], [ %inc, %continue ]
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%p.addr.04 = phi ptr [ %p, %for.body.lr.ph ], [ %incdec.ptr, %continue ]
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%incdec.ptr = getelementptr inbounds i32, ptr %p.addr.04, i32 1
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store i32 %i.05, ptr %p.addr.04, align 4
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%inc = add nsw i32 %i.05, 1
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%cmp = icmp slt i32 %inc, %k
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br i1 %c, label %continue, label %side_exit, !prof !17
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continue:
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br i1 %cmp, label %for.body, label %for.cond.for.end_crit_edge, !prof !16
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for.cond.for.end_crit_edge: ; preds = %for.body
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br label %for.end
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for.end: ; preds = %for.cond.for.end_crit_edge, %entry
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%res = phi i32 [ 0, %entry ], [ %inc, %for.cond.for.end_crit_edge ]
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ret i32 %res
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side_exit:
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%rval = call i32(...) @llvm.experimental.deoptimize.i32() [ "deopt"(i32 %inc) ]
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ret i32 %rval
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}
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declare i32 @llvm.experimental.deoptimize.i32(...)
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attributes #0 = { nounwind }
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attributes #1 = { nounwind optsize }
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!llvm.module.flags = !{!1}
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!1 = !{i32 1, !"ProfileSummary", !2}
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!2 = !{!3, !4, !5, !6, !7, !8, !9, !10}
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!3 = !{!"ProfileFormat", !"InstrProf"}
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!4 = !{!"TotalCount", i64 10}
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!5 = !{!"MaxCount", i64 3}
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!6 = !{!"MaxInternalCount", i64 1}
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!7 = !{!"MaxFunctionCount", i64 3}
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!8 = !{!"NumCounts", i64 2}
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!9 = !{!"NumFunctions", i64 2}
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!10 = !{!"DetailedSummary", !11}
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!11 = !{!12, !13, !14}
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!12 = !{i32 10000, i64 3, i32 2}
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!13 = !{i32 999000, i64 1, i32 10}
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!14 = !{i32 999999, i64 1, i32 10}
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!15 = !{!"function_entry_count", i64 1}
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!16 = !{!"branch_weights", i32 3001, i32 1001}
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!17 = !{!"branch_weights", i32 1, i32 0}
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; This is a weights of deopt side-exit.
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;CHECK: !15 = !{!"branch_weights", i32 1, i32 0}
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; This is a weights of latch and its copies.
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;CHECK: !16 = !{!"branch_weights", i32 3001, i32 1001}
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;CHECK: !17 = !{!"branch_weights", i32 2000, i32 1001}
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;CHECK: !18 = !{!"branch_weights", i32 1001, i32 1001}
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