b05a55787a
Summary: When simplifying unconditional branches from empty blocks, we pre-test if the BB belongs to a set of loop headers and keep the block to prevent passes from destroying canonical loop structure. However, the current algorithm fails if the destination of the branch is a loop header. Especially when such a loop's latch block is folded into loop header it results in additional backedges and LoopSimplify turns it into a nested loop which prevent later optimizations from being applied (e.g., loop unrolling and loop interleaving). This patch augments the existing algorithm by further checking if the destination of the branch belongs to a set of loop headers and defer eliminating it if yes to LateSimplifyCFG. Fixes PR33605: https://bugs.llvm.org/show_bug.cgi?id=33605 Reviewers: efriedma, mcrosier, pacxx, hsung, davidxl Reviewed By: efriedma Subscribers: ashutosh.nema, gberry, javed.absar, llvm-commits Differential Revision: https://reviews.llvm.org/D35411 llvm-svn: 308422
120 lines
3.4 KiB
LLVM
120 lines
3.4 KiB
LLVM
; RUN: opt -S -jump-threading < %s | FileCheck %s
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; Check that based solely on static profile estimation we don't update the
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; branch-weight metadata. Even if the function has an entry frequency, a
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; completely cold part of the CFG may be statically estimated.
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; For example in the loop below, jump threading would update the weight of the
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; loop-exiting branch to 0, drastically inflating the frequency of the loop
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; (in the range of billions).
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;
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; This is the CFG of the loop. There is no run-time profile info for edges
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; inside the loop, so branch and block frequencies are estimated as shown:
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;
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; check_1 (16)
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; (8) / |
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; eq_1 | (8)
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; \ |
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; check_2 (16)
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; (8) / |
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; eq_2 | (8)
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; \ |
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; check_3 (16)
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; (1) / |
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; (loop exit) | (15)
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; |
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; (back edge)
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;
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; First we thread eq_1->check_2 to check_3. Frequencies are updated to remove
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; the frequency of eq_1 from check_2 and then the false edge leaving check_2
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; (changed frequencies are highlighted with * *):
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;
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; check_1 (16)
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; (8) / |
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; eq_1~ | (8)
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; / |
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; / check_2 (*8*)
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; / (8) / |
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; \ eq_2 | (*0*)
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; \ \ |
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; ` --- check_3 (16)
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; (1) / |
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; (loop exit) | (15)
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; |
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; (back edge)
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;
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; Next we thread eq_1->check_3 and eq_2->check_3 to check_1 as new back edges.
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; Frequencies are updated to remove the frequency of eq_1 and eq_3 from
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; check_3 and then the false edge leaving check_3 (changed frequencies are
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; highlighted with * *):
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;
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; check_1 (16)
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; (8) / |
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; eq_1~ | (8)
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; / |
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; / check_2 (*8*)
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; / (8) / |
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; /-- eq_2~ | (*0*)
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; (back edge) |
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; check_3 (*0*)
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; (*0*) / |
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; (loop exit) | (*0*)
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; |
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; (back edge)
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;
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; As a result, the loop exit edge ends up with 0 frequency which in turn makes
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; the loop header to have maximum frequency.
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declare void @bar()
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define void @foo(i32 *%p, i32 %n) !prof !0 {
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entry:
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%enter_loop = icmp eq i32 %n, 0
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br i1 %enter_loop, label %exit, label %check_1, !prof !1
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; CHECK: br i1 %enter_loop, label %exit, label %check_1, !prof !1
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check_1:
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%v = load i32, i32* %p
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%cond1 = icmp eq i32 %v, 1
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br i1 %cond1, label %eq_1, label %check_2
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; No metadata:
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; CHECK: br i1 %cond1, label %check_2.thread, label %check_2{{$}}
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eq_1:
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call void @bar()
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br label %check_2
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; Verify the new backedge:
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; CHECK: check_2.thread:
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; CHECK-NEXT: call void @bar()
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; CHECK-NEXT: br label %check_3.thread
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check_2:
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%cond2 = icmp eq i32 %v, 2
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br i1 %cond2, label %eq_2, label %check_3
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; No metadata:
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; CHECK: br i1 %cond2, label %eq_2, label %check_3{{$}}
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eq_2:
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call void @bar()
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br label %check_3
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; Verify the new backedge:
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; CHECK: eq_2:
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; CHECK-NEXT: call void @bar()
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; CHECK-NEXT: br label %check_3.thread
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check_3:
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%condE = icmp eq i32 %v, 3
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br i1 %condE, label %exit, label %check_1
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; No metadata:
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; CHECK: br i1 %condE, label %exit, label %check_1{{$}}
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exit:
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ret void
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}
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!0 = !{!"function_entry_count", i64 120}
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; CHECK-NOT: branch_weights
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!1 = !{!"branch_weights", i32 119, i32 1}
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; CHECK: !1 = !{!"branch_weights", i32 119, i32 1}
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; CHECK-NOT: branch_weights
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