[SimpleLoopUnswitch] adding cost multiplier to cap exponential unswitch with

We need to control exponential behavior of loop-unswitch so we do not get
run-away compilation.

Suggested solution is to introduce a multiplier for an unswitch cost that
makes cost prohibitive as soon as there are too many candidates and too
many sibling loops (meaning we have already started duplicating loops
by unswitching).

It does solve the currently known problem with compile-time degradation
(PR 39544).

Tests are built on top of a recently implemented CHECK-COUNT-<num>
FileCheck directives.

Reviewed By: chandlerc, mkazantsev
Differential Revision: https://reviews.llvm.org/D54223

llvm-svn: 347097

1 files changed, 80 insertions, 0 deletions

diff --git a/llvm/test/Transforms/SimpleLoopUnswitch/exponential-nontrivial-unswitch.ll b/llvm/test/Transforms/SimpleLoopUnswitch/exponential-nontrivial-unswitch.ll
new file mode 100644
index 00000000000..d013c4f6362
--- /dev/null
+++ b/llvm/test/Transforms/SimpleLoopUnswitch/exponential-nontrivial-unswitch.ll
@@ -0,0 +1,80 @@
+;
+; There should be just a single copy of loop when strictest mutiplier candidates
+; formula (unscaled candidates == 0) is enforced:
+;
+; RUN: opt < %s -enable-nontrivial-unswitch -enable-unswitch-cost-multiplier=true \
+; RUN:     -unswitch-num-initial-unscaled-candidates=0 -unswitch-siblings-toplevel-div=1 \
+; RUN:     -passes='loop(unswitch),print<loops>' -disable-output 2>&1 | FileCheck %s --check-prefixes=LOOP1
+;
+; RUN: opt < %s -enable-nontrivial-unswitch -enable-unswitch-cost-multiplier=true \
+; RUN:     -unswitch-num-initial-unscaled-candidates=0 -unswitch-siblings-toplevel-div=8 \
+; RUN:     -passes='loop(unswitch),print<loops>' -disable-output 2>&1 | FileCheck %s --check-prefixes=LOOP1
+;
+; With relaxed candidates multiplier (unscaled candidates == 8) we should allow
+; some unswitches to happen until siblings multiplier starts kicking in:
+;
+; RUN: opt < %s -enable-nontrivial-unswitch -enable-unswitch-cost-multiplier=true \
+; RUN:     -unswitch-num-initial-unscaled-candidates=8 -unswitch-siblings-toplevel-div=1 \
+; RUN:     -passes='loop(unswitch),print<loops>' -disable-output 2>&1 | FileCheck %s --check-prefixes=LOOP5
+;
+; With relaxed candidates multiplier (unscaled candidates == 8) and with relaxed
+; siblings multiplier for top-level loops (toplevel-div == 8) we should get
+;    2^(num conds) == 2^5 == 32
+; copies of the loop:
+;
+; RUN: opt < %s -enable-nontrivial-unswitch -enable-unswitch-cost-multiplier=true \
+; RUN:     -unswitch-num-initial-unscaled-candidates=8 -unswitch-siblings-toplevel-div=8 \
+; RUN:     -passes='loop(unswitch),print<loops>' -disable-output 2>&1 | FileCheck %s --check-prefixes=LOOP32
+;
+; Similarly get
+;    2^(num conds) == 2^5 == 32
+; copies of the loop when cost multiplier is disabled:
+;
+; RUN: opt < %s -enable-nontrivial-unswitch -enable-unswitch-cost-multiplier=false \
+; RUN:     -passes='loop(unswitch),print<loops>' -disable-output 2>&1 | FileCheck %s --check-prefixes=LOOP32
+;
+;
+; Single loop, not unswitched
+; LOOP1:     Loop at depth 1 containing:
+; LOOP1-NOT: Loop at depth 1 containing:
+
+; 5 loops, unswitched 4 times
+; LOOP5-COUNT-5: Loop at depth 1 containing:
+; LOOP5-NOT:     Loop at depth 1 containing:
+
+; 32 loops, fully unswitched
+; LOOP32-COUNT-32: Loop at depth 1 containing:
+; LOOP32-NOT:     Loop at depth 1 containing:
+
+define void @loop_simple5(i32* %addr, i1 %c1, i1 %c2, i1 %c3, i1 %c4, i1 %c5) {
+entry:
+  br label %loop
+loop:
+  %iv = phi i32 [0, %entry], [%iv.next, %loop_latch]
+  %iv.next = add i32 %iv, 1
+  br i1 %c1, label %loop_next1, label %loop_next1_right
+loop_next1_right:
+  br label %loop_next1
+loop_next1:
+  br i1 %c2, label %loop_next2, label %loop_next2_right
+loop_next2_right:
+  br label %loop_next2
+loop_next2:
+  br i1 %c3, label %loop_next3, label %loop_next3_right
+loop_next3_right:
+  br label %loop_next3
+loop_next3:
+  br i1 %c4, label %loop_next4, label %loop_next4_right
+loop_next4_right:
+  br label %loop_next4
+loop_next4:
+  br i1 %c5, label %loop_latch, label %loop_latch_right
+loop_latch_right:
+  br label %loop_latch
+loop_latch:
+  store volatile i32 0, i32* %addr
+  %test_loop = icmp slt i32 %iv, 50
+  br i1 %test_loop, label %loop, label %exit
+exit:
+  ret void
+}