Chandler Carruth 4da3331d3d [PM/LoopUnswitch] Support partial trivial unswitching.
The idea of partial unswitching is to take a *part* of a branch's
condition that is loop invariant and just unswitching that part. This
primarily makes sense with i1 conditions of branches as opposed to
switches. When dealing with i1 conditions, we can easily extract loop
invariant inputs to a a branch and unswitch them to test them entirely
outside the loop.

As part of this, we now create much more significant cruft in the loop
body, so this relies on adding cleanup passes to the loop pipeline and
revisiting unswitched loops to do that cleanup before continuing to
process them.

This already appears to be more powerful at unswitching than the old
loop unswitch pass, and so I'd appreciate pretty careful review in case
I'm just missing some correctness checks. The `LIV-loop-condition` test
case is not unswitched by the old unswitch pass, but is with this pass.

Thanks to Sanjoy and Fedor for the review!

Differential Revision: https://reviews.llvm.org/D46706

llvm-svn: 335156
2018-06-20 18:57:07 +00:00

622 lines
18 KiB
LLVM

; RUN: opt -passes='loop(unswitch),verify<loops>' -S < %s | FileCheck %s
declare void @some_func() noreturn
; This test contains two trivial unswitch condition in one loop.
; LoopUnswitch pass should be able to unswitch the second one
; after unswitching the first one.
define i32 @test1(i32* %var, i1 %cond1, i1 %cond2) {
; CHECK-LABEL: @test1(
entry:
br label %loop_begin
; CHECK-NEXT: entry:
; CHECK-NEXT: br i1 %{{.*}}, label %entry.split, label %loop_exit.split
;
; CHECK: entry.split:
; CHECK-NEXT: br i1 %{{.*}}, label %entry.split.split, label %loop_exit
;
; CHECK: entry.split.split:
; CHECK-NEXT: br label %loop_begin
loop_begin:
br i1 %cond1, label %continue, label %loop_exit ; first trivial condition
; CHECK: loop_begin:
; CHECK-NEXT: br label %continue
continue:
%var_val = load i32, i32* %var
br i1 %cond2, label %do_something, label %loop_exit ; second trivial condition
; CHECK: continue:
; CHECK-NEXT: load
; CHECK-NEXT: br label %do_something
do_something:
call void @some_func() noreturn nounwind
br label %loop_begin
; CHECK: do_something:
; CHECK-NEXT: call
; CHECK-NEXT: br label %loop_begin
loop_exit:
ret i32 0
; CHECK: loop_exit:
; CHECK-NEXT: br label %loop_exit.split
;
; CHECK: loop_exit.split:
; CHECK-NEXT: ret
}
; Test for two trivially unswitchable switches.
define i32 @test3(i32* %var, i32 %cond1, i32 %cond2) {
; CHECK-LABEL: @test3(
entry:
br label %loop_begin
; CHECK-NEXT: entry:
; CHECK-NEXT: switch i32 %cond1, label %entry.split [
; CHECK-NEXT: i32 0, label %loop_exit1
; CHECK-NEXT: ]
;
; CHECK: entry.split:
; CHECK-NEXT: switch i32 %cond2, label %loop_exit2 [
; CHECK-NEXT: i32 42, label %loop_exit2
; CHECK-NEXT: i32 0, label %entry.split.split
; CHECK-NEXT: ]
;
; CHECK: entry.split.split:
; CHECK-NEXT: br label %loop_begin
loop_begin:
switch i32 %cond1, label %continue [
i32 0, label %loop_exit1
]
; CHECK: loop_begin:
; CHECK-NEXT: br label %continue
continue:
%var_val = load i32, i32* %var
switch i32 %cond2, label %loop_exit2 [
i32 0, label %do_something
i32 42, label %loop_exit2
]
; CHECK: continue:
; CHECK-NEXT: load
; CHECK-NEXT: br label %do_something
do_something:
call void @some_func() noreturn nounwind
br label %loop_begin
; CHECK: do_something:
; CHECK-NEXT: call
; CHECK-NEXT: br label %loop_begin
loop_exit1:
ret i32 0
; CHECK: loop_exit1:
; CHECK-NEXT: ret
loop_exit2:
ret i32 0
; CHECK: loop_exit2:
; CHECK-NEXT: ret
;
; We shouldn't have any unreachable blocks here because the unswitched switches
; turn into branches instead.
; CHECK-NOT: unreachable
}
; Test for a trivially unswitchable switch with multiple exiting cases and
; multiple looping cases.
define i32 @test4(i32* %var, i32 %cond1, i32 %cond2) {
; CHECK-LABEL: @test4(
entry:
br label %loop_begin
; CHECK-NEXT: entry:
; CHECK-NEXT: switch i32 %cond2, label %loop_exit2 [
; CHECK-NEXT: i32 13, label %loop_exit1
; CHECK-NEXT: i32 42, label %loop_exit3
; CHECK-NEXT: i32 0, label %entry.split
; CHECK-NEXT: i32 1, label %entry.split
; CHECK-NEXT: i32 2, label %entry.split
; CHECK-NEXT: ]
;
; CHECK: entry.split:
; CHECK-NEXT: br label %loop_begin
loop_begin:
%var_val = load i32, i32* %var
switch i32 %cond2, label %loop_exit2 [
i32 0, label %loop0
i32 1, label %loop1
i32 13, label %loop_exit1
i32 2, label %loop2
i32 42, label %loop_exit3
]
; CHECK: loop_begin:
; CHECK-NEXT: load
; CHECK-NEXT: switch i32 %cond2, label %[[UNREACHABLE:.*]] [
; CHECK-NEXT: i32 0, label %loop0
; CHECK-NEXT: i32 1, label %loop1
; CHECK-NEXT: i32 2, label %loop2
; CHECK-NEXT: ]
loop0:
call void @some_func() noreturn nounwind
br label %loop_latch
; CHECK: loop0:
; CHECK-NEXT: call
; CHECK-NEXT: br label %loop_latch
loop1:
call void @some_func() noreturn nounwind
br label %loop_latch
; CHECK: loop1:
; CHECK-NEXT: call
; CHECK-NEXT: br label %loop_latch
loop2:
call void @some_func() noreturn nounwind
br label %loop_latch
; CHECK: loop2:
; CHECK-NEXT: call
; CHECK-NEXT: br label %loop_latch
loop_latch:
br label %loop_begin
; CHECK: loop_latch:
; CHECK-NEXT: br label %loop_begin
loop_exit1:
ret i32 0
; CHECK: loop_exit1:
; CHECK-NEXT: ret
loop_exit2:
ret i32 0
; CHECK: loop_exit2:
; CHECK-NEXT: ret
loop_exit3:
ret i32 0
; CHECK: loop_exit3:
; CHECK-NEXT: ret
;
; CHECK: [[UNREACHABLE]]:
; CHECK-NEXT: unreachable
}
; This test contains a trivially unswitchable branch with an LCSSA phi node in
; a loop exit block.
define i32 @test5(i1 %cond1, i32 %x, i32 %y) {
; CHECK-LABEL: @test5(
entry:
br label %loop_begin
; CHECK-NEXT: entry:
; CHECK-NEXT: br i1 %{{.*}}, label %entry.split, label %loop_exit
;
; CHECK: entry.split:
; CHECK-NEXT: br label %loop_begin
loop_begin:
br i1 %cond1, label %latch, label %loop_exit
; CHECK: loop_begin:
; CHECK-NEXT: br label %latch
latch:
call void @some_func() noreturn nounwind
br label %loop_begin
; CHECK: latch:
; CHECK-NEXT: call
; CHECK-NEXT: br label %loop_begin
loop_exit:
%result1 = phi i32 [ %x, %loop_begin ]
%result2 = phi i32 [ %y, %loop_begin ]
%result = add i32 %result1, %result2
ret i32 %result
; CHECK: loop_exit:
; CHECK-NEXT: %[[R1:.*]] = phi i32 [ %x, %entry ]
; CHECK-NEXT: %[[R2:.*]] = phi i32 [ %y, %entry ]
; CHECK-NEXT: %[[R:.*]] = add i32 %[[R1]], %[[R2]]
; CHECK-NEXT: ret i32 %[[R]]
}
; This test contains a trivially unswitchable branch with a real phi node in LCSSA
; position in a shared exit block where a different path through the loop
; produces a non-invariant input to the PHI node.
define i32 @test6(i32* %var, i1 %cond1, i1 %cond2, i32 %x, i32 %y) {
; CHECK-LABEL: @test6(
entry:
br label %loop_begin
; CHECK-NEXT: entry:
; CHECK-NEXT: br i1 %{{.*}}, label %entry.split, label %loop_exit.split
;
; CHECK: entry.split:
; CHECK-NEXT: br label %loop_begin
loop_begin:
br i1 %cond1, label %continue, label %loop_exit
; CHECK: loop_begin:
; CHECK-NEXT: br label %continue
continue:
%var_val = load i32, i32* %var
br i1 %cond2, label %latch, label %loop_exit
; CHECK: continue:
; CHECK-NEXT: load
; CHECK-NEXT: br i1 %cond2, label %latch, label %loop_exit
latch:
call void @some_func() noreturn nounwind
br label %loop_begin
; CHECK: latch:
; CHECK-NEXT: call
; CHECK-NEXT: br label %loop_begin
loop_exit:
%result1 = phi i32 [ %x, %loop_begin ], [ %var_val, %continue ]
%result2 = phi i32 [ %var_val, %continue ], [ %y, %loop_begin ]
%result = add i32 %result1, %result2
ret i32 %result
; CHECK: loop_exit:
; CHECK-NEXT: %[[R1:.*]] = phi i32 [ %var_val, %continue ]
; CHECK-NEXT: %[[R2:.*]] = phi i32 [ %var_val, %continue ]
; CHECK-NEXT: br label %loop_exit.split
;
; CHECK: loop_exit.split:
; CHECK-NEXT: %[[R1S:.*]] = phi i32 [ %x, %entry ], [ %[[R1]], %loop_exit ]
; CHECK-NEXT: %[[R2S:.*]] = phi i32 [ %y, %entry ], [ %[[R2]], %loop_exit ]
; CHECK-NEXT: %[[R:.*]] = add i32 %[[R1S]], %[[R2S]]
; CHECK-NEXT: ret i32 %[[R]]
}
; This test contains a trivially unswitchable switch with an LCSSA phi node in
; a loop exit block.
define i32 @test7(i32 %cond1, i32 %x, i32 %y) {
; CHECK-LABEL: @test7(
entry:
br label %loop_begin
; CHECK-NEXT: entry:
; CHECK-NEXT: switch i32 %cond1, label %entry.split [
; CHECK-NEXT: i32 0, label %loop_exit
; CHECK-NEXT: i32 1, label %loop_exit
; CHECK-NEXT: ]
;
; CHECK: entry.split:
; CHECK-NEXT: br label %loop_begin
loop_begin:
switch i32 %cond1, label %latch [
i32 0, label %loop_exit
i32 1, label %loop_exit
]
; CHECK: loop_begin:
; CHECK-NEXT: br label %latch
latch:
call void @some_func() noreturn nounwind
br label %loop_begin
; CHECK: latch:
; CHECK-NEXT: call
; CHECK-NEXT: br label %loop_begin
loop_exit:
%result1 = phi i32 [ %x, %loop_begin ], [ %x, %loop_begin ]
%result2 = phi i32 [ %y, %loop_begin ], [ %y, %loop_begin ]
%result = add i32 %result1, %result2
ret i32 %result
; CHECK: loop_exit:
; CHECK-NEXT: %[[R1:.*]] = phi i32 [ %x, %entry ], [ %x, %entry ]
; CHECK-NEXT: %[[R2:.*]] = phi i32 [ %y, %entry ], [ %y, %entry ]
; CHECK-NEXT: %[[R:.*]] = add i32 %[[R1]], %[[R2]]
; CHECK-NEXT: ret i32 %[[R]]
}
; This test contains a trivially unswitchable switch with a real phi node in
; LCSSA position in a shared exit block where a different path through the loop
; produces a non-invariant input to the PHI node.
define i32 @test8(i32* %var, i32 %cond1, i32 %cond2, i32 %x, i32 %y) {
; CHECK-LABEL: @test8(
entry:
br label %loop_begin
; CHECK-NEXT: entry:
; CHECK-NEXT: switch i32 %cond1, label %entry.split [
; CHECK-NEXT: i32 0, label %loop_exit.split
; CHECK-NEXT: i32 1, label %loop_exit2
; CHECK-NEXT: i32 2, label %loop_exit.split
; CHECK-NEXT: ]
;
; CHECK: entry.split:
; CHECK-NEXT: br label %loop_begin
loop_begin:
switch i32 %cond1, label %continue [
i32 0, label %loop_exit
i32 1, label %loop_exit2
i32 2, label %loop_exit
]
; CHECK: loop_begin:
; CHECK-NEXT: br label %continue
continue:
%var_val = load i32, i32* %var
switch i32 %cond2, label %latch [
i32 0, label %loop_exit
]
; CHECK: continue:
; CHECK-NEXT: load
; CHECK-NEXT: switch i32 %cond2, label %latch [
; CHECK-NEXT: i32 0, label %loop_exit
; CHECK-NEXT: ]
latch:
call void @some_func() noreturn nounwind
br label %loop_begin
; CHECK: latch:
; CHECK-NEXT: call
; CHECK-NEXT: br label %loop_begin
loop_exit:
%result1.1 = phi i32 [ %x, %loop_begin ], [ %x, %loop_begin ], [ %var_val, %continue ]
%result1.2 = phi i32 [ %var_val, %continue ], [ %y, %loop_begin ], [ %y, %loop_begin ]
%result1 = add i32 %result1.1, %result1.2
ret i32 %result1
; CHECK: loop_exit:
; CHECK-NEXT: %[[R1:.*]] = phi i32 [ %var_val, %continue ]
; CHECK-NEXT: %[[R2:.*]] = phi i32 [ %var_val, %continue ]
; CHECK-NEXT: br label %loop_exit.split
;
; CHECK: loop_exit.split:
; CHECK-NEXT: %[[R1S:.*]] = phi i32 [ %x, %entry ], [ %x, %entry ], [ %[[R1]], %loop_exit ]
; CHECK-NEXT: %[[R2S:.*]] = phi i32 [ %y, %entry ], [ %y, %entry ], [ %[[R2]], %loop_exit ]
; CHECK-NEXT: %[[R:.*]] = add i32 %[[R1S]], %[[R2S]]
; CHECK-NEXT: ret i32 %[[R]]
loop_exit2:
%result2.1 = phi i32 [ %x, %loop_begin ]
%result2.2 = phi i32 [ %y, %loop_begin ]
%result2 = add i32 %result2.1, %result2.2
ret i32 %result2
; CHECK: loop_exit2:
; CHECK-NEXT: %[[R1:.*]] = phi i32 [ %x, %entry ]
; CHECK-NEXT: %[[R2:.*]] = phi i32 [ %y, %entry ]
; CHECK-NEXT: %[[R:.*]] = add i32 %[[R1]], %[[R2]]
; CHECK-NEXT: ret i32 %[[R]]
}
; This test, extracted from the LLVM test suite, has an interesting dominator
; tree to update as there are edges to sibling domtree nodes within child
; domtree nodes of the unswitched node.
define void @xgets(i1 %cond1, i1* %cond2.ptr) {
; CHECK-LABEL: @xgets(
entry:
br label %for.cond.preheader
; CHECK: entry:
; CHECK-NEXT: br label %for.cond.preheader
for.cond.preheader:
br label %for.cond
; CHECK: for.cond.preheader:
; CHECK-NEXT: br i1 %cond1, label %for.cond.preheader.split, label %if.end17.thread.loopexit
;
; CHECK: for.cond.preheader.split:
; CHECK-NEXT: br label %for.cond
for.cond:
br i1 %cond1, label %land.lhs.true, label %if.end17.thread.loopexit
; CHECK: for.cond:
; CHECK-NEXT: br label %land.lhs.true
land.lhs.true:
br label %if.then20
; CHECK: land.lhs.true:
; CHECK-NEXT: br label %if.then20
if.then20:
%cond2 = load volatile i1, i1* %cond2.ptr
br i1 %cond2, label %if.then23, label %if.else
; CHECK: if.then20:
; CHECK-NEXT: %[[COND2:.*]] = load volatile i1, i1* %cond2.ptr
; CHECK-NEXT: br i1 %[[COND2]], label %if.then23, label %if.else
if.else:
br label %for.cond
; CHECK: if.else:
; CHECK-NEXT: br label %for.cond
if.end17.thread.loopexit:
br label %if.end17.thread
; CHECK: if.end17.thread.loopexit:
; CHECK-NEXT: br label %if.end17.thread
if.end17.thread:
br label %cleanup
; CHECK: if.end17.thread:
; CHECK-NEXT: br label %cleanup
if.then23:
br label %cleanup
; CHECK: if.then23:
; CHECK-NEXT: br label %cleanup
cleanup:
ret void
; CHECK: cleanup:
; CHECK-NEXT: ret void
}
define i32 @test_partial_condition_unswitch_and(i32* %var, i1 %cond1, i1 %cond2) {
; CHECK-LABEL: @test_partial_condition_unswitch_and(
entry:
br label %loop_begin
; CHECK-NEXT: entry:
; CHECK-NEXT: br i1 %cond1, label %entry.split, label %loop_exit.split
;
; CHECK: entry.split:
; CHECK-NEXT: br i1 %cond2, label %entry.split.split, label %loop_exit
;
; CHECK: entry.split.split:
; CHECK-NEXT: br label %loop_begin
loop_begin:
br i1 %cond1, label %continue, label %loop_exit
; CHECK: loop_begin:
; CHECK-NEXT: br label %continue
continue:
%var_val = load i32, i32* %var
%var_cond = trunc i32 %var_val to i1
%cond_and = and i1 %var_cond, %cond2
br i1 %cond_and, label %do_something, label %loop_exit
; CHECK: continue:
; CHECK-NEXT: %[[VAR:.*]] = load i32
; CHECK-NEXT: %[[VAR_COND:.*]] = trunc i32 %[[VAR]] to i1
; CHECK-NEXT: %[[COND_AND:.*]] = and i1 %[[VAR_COND]], true
; CHECK-NEXT: br i1 %[[COND_AND]], label %do_something, label %loop_exit
do_something:
call void @some_func() noreturn nounwind
br label %loop_begin
; CHECK: do_something:
; CHECK-NEXT: call
; CHECK-NEXT: br label %loop_begin
loop_exit:
ret i32 0
; CHECK: loop_exit:
; CHECK-NEXT: br label %loop_exit.split
;
; CHECK: loop_exit.split:
; CHECK-NEXT: ret
}
define i32 @test_partial_condition_unswitch_or(i32* %var, i1 %cond1, i1 %cond2, i1 %cond3, i1 %cond4, i1 %cond5, i1 %cond6) {
; CHECK-LABEL: @test_partial_condition_unswitch_or(
entry:
br label %loop_begin
; CHECK-NEXT: entry:
; CHECK-NEXT: %[[INV_OR1:.*]] = or i1 %cond4, %cond2
; CHECK-NEXT: %[[INV_OR2:.*]] = or i1 %[[INV_OR1]], %cond3
; CHECK-NEXT: %[[INV_OR3:.*]] = or i1 %[[INV_OR2]], %cond1
; CHECK-NEXT: br i1 %[[INV_OR3]], label %loop_exit.split, label %entry.split
;
; CHECK: entry.split:
; CHECK-NEXT: br label %loop_begin
loop_begin:
%var_val = load i32, i32* %var
%var_cond = trunc i32 %var_val to i1
%cond_or1 = or i1 %var_cond, %cond1
%cond_or2 = or i1 %cond2, %cond3
%cond_or3 = or i1 %cond_or1, %cond_or2
%cond_xor1 = xor i1 %cond5, %var_cond
%cond_and1 = and i1 %cond6, %var_cond
%cond_or4 = or i1 %cond_xor1, %cond_and1
%cond_or5 = or i1 %cond_or3, %cond_or4
%cond_or6 = or i1 %cond_or5, %cond4
br i1 %cond_or6, label %loop_exit, label %do_something
; CHECK: loop_begin:
; CHECK-NEXT: %[[VAR:.*]] = load i32
; CHECK-NEXT: %[[VAR_COND:.*]] = trunc i32 %[[VAR]] to i1
; CHECK-NEXT: %[[COND_OR1:.*]] = or i1 %[[VAR_COND]], false
; CHECK-NEXT: %[[COND_OR2:.*]] = or i1 false, false
; CHECK-NEXT: %[[COND_OR3:.*]] = or i1 %[[COND_OR1]], %[[COND_OR2]]
; CHECK-NEXT: %[[COND_XOR:.*]] = xor i1 %cond5, %[[VAR_COND]]
; CHECK-NEXT: %[[COND_AND:.*]] = and i1 %cond6, %[[VAR_COND]]
; CHECK-NEXT: %[[COND_OR4:.*]] = or i1 %[[COND_XOR]], %[[COND_AND]]
; CHECK-NEXT: %[[COND_OR5:.*]] = or i1 %[[COND_OR3]], %[[COND_OR4]]
; CHECK-NEXT: %[[COND_OR6:.*]] = or i1 %[[COND_OR5]], false
; CHECK-NEXT: br i1 %[[COND_OR6]], label %loop_exit, label %do_something
do_something:
call void @some_func() noreturn nounwind
br label %loop_begin
; CHECK: do_something:
; CHECK-NEXT: call
; CHECK-NEXT: br label %loop_begin
loop_exit:
ret i32 0
; CHECK: loop_exit.split:
; CHECK-NEXT: ret
}
define i32 @test_partial_condition_unswitch_with_lcssa_phi1(i32* %var, i1 %cond, i32 %x) {
; CHECK-LABEL: @test_partial_condition_unswitch_with_lcssa_phi1(
entry:
br label %loop_begin
; CHECK-NEXT: entry:
; CHECK-NEXT: br i1 %cond, label %entry.split, label %loop_exit.split
;
; CHECK: entry.split:
; CHECK-NEXT: br label %loop_begin
loop_begin:
%var_val = load i32, i32* %var
%var_cond = trunc i32 %var_val to i1
%cond_and = and i1 %var_cond, %cond
br i1 %cond_and, label %do_something, label %loop_exit
; CHECK: loop_begin:
; CHECK-NEXT: %[[VAR:.*]] = load i32
; CHECK-NEXT: %[[VAR_COND:.*]] = trunc i32 %[[VAR]] to i1
; CHECK-NEXT: %[[COND_AND:.*]] = and i1 %[[VAR_COND]], true
; CHECK-NEXT: br i1 %[[COND_AND]], label %do_something, label %loop_exit
do_something:
call void @some_func() noreturn nounwind
br label %loop_begin
; CHECK: do_something:
; CHECK-NEXT: call
; CHECK-NEXT: br label %loop_begin
loop_exit:
%x.lcssa = phi i32 [ %x, %loop_begin ]
ret i32 %x.lcssa
; CHECK: loop_exit:
; CHECK-NEXT: %[[LCSSA:.*]] = phi i32 [ %x, %loop_begin ]
; CHECK-NEXT: br label %loop_exit.split
;
; CHECK: loop_exit.split:
; CHECK-NEXT: %[[LCSSA_SPLIT:.*]] = phi i32 [ %x, %entry ], [ %[[LCSSA]], %loop_exit ]
; CHECK-NEXT: ret i32 %[[LCSSA_SPLIT]]
}
define i32 @test_partial_condition_unswitch_with_lcssa_phi2(i32* %var, i1 %cond, i32 %x, i32 %y) {
; CHECK-LABEL: @test_partial_condition_unswitch_with_lcssa_phi2(
entry:
br label %loop_begin
; CHECK-NEXT: entry:
; CHECK-NEXT: br i1 %cond, label %entry.split, label %loop_exit.split
;
; CHECK: entry.split:
; CHECK-NEXT: br label %loop_begin
loop_begin:
%var_val = load i32, i32* %var
%var_cond = trunc i32 %var_val to i1
%cond_and = and i1 %var_cond, %cond
br i1 %cond_and, label %do_something, label %loop_exit
; CHECK: loop_begin:
; CHECK-NEXT: %[[VAR:.*]] = load i32
; CHECK-NEXT: %[[VAR_COND:.*]] = trunc i32 %[[VAR]] to i1
; CHECK-NEXT: %[[COND_AND:.*]] = and i1 %[[VAR_COND]], true
; CHECK-NEXT: br i1 %[[COND_AND]], label %do_something, label %loop_exit
do_something:
call void @some_func() noreturn nounwind
br i1 %var_cond, label %loop_begin, label %loop_exit
; CHECK: do_something:
; CHECK-NEXT: call
; CHECK-NEXT: br i1 %[[VAR_COND]], label %loop_begin, label %loop_exit
loop_exit:
%xy.lcssa = phi i32 [ %x, %loop_begin ], [ %y, %do_something ]
ret i32 %xy.lcssa
; CHECK: loop_exit:
; CHECK-NEXT: %[[LCSSA:.*]] = phi i32 [ %x, %loop_begin ], [ %y, %do_something ]
; CHECK-NEXT: br label %loop_exit.split
;
; CHECK: loop_exit.split:
; CHECK-NEXT: %[[LCSSA_SPLIT:.*]] = phi i32 [ %x, %entry ], [ %[[LCSSA]], %loop_exit ]
; CHECK-NEXT: ret i32 %[[LCSSA_SPLIT]]
}