Enable loop rotate when the header is exiting

This was recently disabled but turns out to be an important case for loops
with preconditions.

Author: meg-gupta

lib/SILOptimizer/LoopTransforms/LoopRotate.cpp

@@ -369,12 +369,6 @@ static bool rotateLoop(SILLoop *loop, DominanceInfo *domInfo,
   assert(loop->contains(newHeader) && !loop->contains(exit)
          && "Could not find loop header and exit block");
 
-  // It does not make sense to rotate the loop if the new header is loop
-  // exiting as well.
-  if (loop->isLoopExiting(newHeader)) {
-    return false;
-  }
-
   // Incomplete liveranges in the dead-end exit block can cause a missing adjacent
   // phi-argument for a re-borrow if there is a borrow-scope is in the loop.
   // But even when we have complete lifetimes, it's probably not worth rotating

test/SILOptimizer/looprotate_ossa.sil

@@ -85,16 +85,13 @@ bb3:
   return %23 : $Int32
 }
 
-// CHECK-LABEL: sil [ossa] @dont_rotate_multi_exit_loop :
-// CHECK:       bb1({{.*}}):
-// CHECK:         cond_br %{{.*}}, bb4, bb2
-// CHECK:       bb2:
-// CHECK:         cond_br undef, bb3, bb5
-// CHECK:       bb6:
-// CHECK:         return
-// CHECK-NOT:   bb7
-// CHECK:       } // end sil function 'dont_rotate_multi_exit_loop'
-sil [ossa] @dont_rotate_multi_exit_loop : $@convention(thin) (Int32, @owned Bar) -> Int32 {
+// CHECK-LABEL: sil [ossa] @rotate_multi_exit_loop :
+// CHECK:  [[METH1:%.*]] = class_method {{.*}} : $Bar, #Bar.foo : (Bar) -> () -> (), $@convention(method) (@guaranteed Bar) -> ()
+// CHECK:  apply [[METH1]]({{.*}}) : $@convention(method) (@guaranteed Bar) -> ()
+// CHECK:  [[METH2:%.*]] = class_method {{.*}} : $Bar, #Bar.foo : (Bar) -> () -> (), $@convention(method) (@guaranteed Bar) -> ()
+// CHECK:  apply [[METH2]]({{.*}}) : $@convention(method) (@guaranteed Bar) -> ()
+// CHECK:  } // end sil function 'rotate_multi_exit_loop'
+sil [ossa] @rotate_multi_exit_loop : $@convention(thin) (Int32, @owned Bar) -> Int32 {
 bb0(%0 : $Int32, %25: @owned $Bar):
   %1 = struct_extract %0 : $Int32, #Int32._value
   %2 = integer_literal $Builtin.Int32, 0

test/SILOptimizer/mutable_span_bounds_check_tests.swift

@@ -11,13 +11,13 @@
 import SpanExtras
 
 // Bounds check should be eliminated
-// SIL does not optimize this
+// SIL removes lower bounds check from the loop
 // LLVM leaves behind a lower bound check outside the loop, does not vectorize the loop
 
 // CHECK-SIL-LABEL: sil @$s31mutable_span_bounds_check_tests0B10_zero_inityy10SpanExtras07MutableH0VySiGzF : 
-// CHECK: bb2:
-// CHECK: cond_fail {{.*}}, "Index out of bounds"
+// CHECK: bb3:
 // CHECK: cond_fail {{.*}}, "Index out of bounds"
+// CHECK-NOT: cond_fail {{.*}}, "Index out of bounds"
 // CHECK: cond_br
 // CHECK-SIL-LABEL: } // end sil function '$s31mutable_span_bounds_check_tests0B10_zero_inityy10SpanExtras07MutableH0VySiGzF'
 public func span_zero_init(_ output: inout MutableSpan<Int>) {
@@ -27,13 +27,13 @@ public func span_zero_init(_ output: inout MutableSpan<Int>) {
 }
 
 // Bounds check should be eliminated
-// SIL does not optimize this
+// SIL removes lower bounds check from the loop
 // LLVM leaves behind a lower bound check outside the loop, does not vectorize the loop or reduce to a memcopy
 
 // CHECK-SIL-LABEL: sil @$s31mutable_span_bounds_check_tests0B14_copy_elemwiseyy10SpanExtras07MutableH0VySiGz_s0H0VySiGtF :
-// CHECK: bb2:
-// CHECK: cond_fail {{.*}}, "Index out of bounds"
+// CHECK: bb3:
 // CHECK: cond_fail {{.*}}, "Index out of bounds"
+// CHECK-NOT: cond_fail {{.*}}, "Index out of bounds"
 // CHECK: cond_br
 // CHECK-SIL-LABEL: } // end sil function '$s31mutable_span_bounds_check_tests0B14_copy_elemwiseyy10SpanExtras07MutableH0VySiGz_s0H0VySiGtF'
 public func span_copy_elemwise(_ output: inout MutableSpan<Int>, _ input: Span<Int>) {
@@ -44,12 +44,12 @@ public func span_copy_elemwise(_ output: inout MutableSpan<Int>, _ input: Span<I
 }
 
 // Bounds check should be eliminated
-// SIL does not optimize this
+// SIL removes lower bounds check from the loop
 
 // CHECK-SIL-LABEL: sil @$s31mutable_span_bounds_check_tests0B16_append_elemwiseyy10SpanExtras06OutputH0VySiGz_s0H0VySiGtF : 
-// CHECK: bb2:
-// CHECK: cond_fail {{.*}}, "Index out of bounds"
+// CHECK: bb3:
 // CHECK: cond_fail {{.*}}, "Index out of bounds"
+// CHECK-NOT: cond_fail {{.*}}, "Index out of bounds"
 // CHECK: cond_br
 // CHECK-SIL-LABEL: } // end sil function '$s31mutable_span_bounds_check_tests0B16_append_elemwiseyy10SpanExtras06OutputH0VySiGz_s0H0VySiGtF'
 public func span_append_elemwise(_ output: inout OutputSpan<Int>, _ input: Span<Int>) {
@@ -59,12 +59,12 @@ public func span_append_elemwise(_ output: inout OutputSpan<Int>, _ input: Span<
 }
 
 // Bounds check should be eliminated
-// SIL does not optimize this
+// SIL removes lower bounds check from the loop
 
 // CHECK-SIL-LABEL: sil @$s31mutable_span_bounds_check_tests0B12_sum_wo_trapyy10SpanExtras07MutableI0VySiGz_s0I0VySiGAItF : 
-// CHECK: bb2:
-// CHECK: cond_fail {{.*}}, "Index out of bounds"
+// CHECK: bb3:
 // CHECK: cond_fail {{.*}}, "Index out of bounds"
+// CHECK-NOT: cond_fail {{.*}}, "Index out of bounds"
 // CHECK: cond_br
 // CHECK-SIL-LABEL: } // end sil function '$s31mutable_span_bounds_check_tests0B12_sum_wo_trapyy10SpanExtras07MutableI0VySiGz_s0I0VySiGAItF'
 public func span_sum_wo_trap(_ output: inout MutableSpan<Int>, _ input1: Span<Int>, _ input2: Span<Int>) {
@@ -76,9 +76,9 @@ public func span_sum_wo_trap(_ output: inout MutableSpan<Int>, _ input1: Span<In
 }
 
 // CHECK-SIL-LABEL: sil @$s31mutable_span_bounds_check_tests0B14_sum_with_trapyy10SpanExtras07MutableI0VySiGz_s0I0VySiGAItF :
-// CHECK: bb2:
-// CHECK: cond_fail {{.*}}, "Index out of bounds"
+// CHECK: bb3:
 // CHECK: cond_fail {{.*}}, "Index out of bounds"
+// CHECK-NOT: cond_fail {{.*}}, "Index out of bounds"
 // CHECK: cond_br
 // CHECK-SIL-LABEL: } // end sil function '$s31mutable_span_bounds_check_tests0B14_sum_with_trapyy10SpanExtras07MutableI0VySiGz_s0I0VySiGAItF'
 public func span_sum_with_trap(_ output: inout MutableSpan<Int>, _ input1: Span<Int>, _ input2: Span<Int>) {