[IRCE] Identify loops with latch comparison against current IV value

Current implementation of parseLoopStructure interprets the latch comparison as a
comarison against `iv.next`. If the actual comparison is made against the `iv` current value
then the loop may be rejected, because this misinterpretation leads to incorrect evaluation
of the latch start value.

This patch teaches the IRCE to distinguish this kind of loops and perform the optimization
for them. Now we use `IndVarBase` variable which can be either next or current value of the
induction variable (previously we used `IndVarNext` which was always the value on next iteration).

Differential Revision: https://reviews.llvm.org/D36215

llvm-svn: 312221

1 files changed, 182 insertions, 0 deletions

diff --git a/llvm/test/Transforms/IRCE/latch-comparison-against-current-value.ll b/llvm/test/Transforms/IRCE/latch-comparison-against-current-value.ll
new file mode 100644
index 00000000000..afea0e68915
--- /dev/null
+++ b/llvm/test/Transforms/IRCE/latch-comparison-against-current-value.ll
@@ -0,0 +1,182 @@
+; RUN: opt -verify-loop-info -irce-print-changed-loops -irce -S < %s 2>&1 | FileCheck %s
+
+; Check that IRCE is able to deal with loops where the latch comparison is
+; done against current value of the IV, not the IV.next.
+
+; CHECK: irce: in function test_01: constrained Loop at depth 1 containing: %loop<header><exiting>,%in.bounds<latch><exiting>
+; CHECK: irce: in function test_02: constrained Loop at depth 1 containing: %loop<header><exiting>,%in.bounds<latch><exiting>
+; CHECK-NOT: irce: in function test_03: constrained Loop at depth 1 containing: %loop<header><exiting>,%in.bounds<latch><exiting>
+; CHECK-NOT: irce: in function test_04: constrained Loop at depth 1 containing: %loop<header><exiting>,%in.bounds<latch><exiting>
+
+; SLT condition for increasing loop from 0 to 100.
+define void @test_01(i32* %arr, i32* %a_len_ptr) #0 {
+
+; CHECK:      test_01
+; CHECK:        entry:
+; CHECK-NEXT:     %exit.mainloop.at = load i32, i32* %a_len_ptr, !range !0
+; CHECK-NEXT:     [[COND2:%[^ ]+]] = icmp slt i32 0, %exit.mainloop.at
+; CHECK-NEXT:     br i1 [[COND2]], label %loop.preheader, label %main.pseudo.exit
+; CHECK:        loop:
+; CHECK-NEXT:     %idx = phi i32 [ %idx.next, %in.bounds ], [ 0, %loop.preheader ]
+; CHECK-NEXT:     %idx.next = add nuw nsw i32 %idx, 1
+; CHECK-NEXT:     %abc = icmp slt i32 %idx, %exit.mainloop.at
+; CHECK-NEXT:     br i1 true, label %in.bounds, label %out.of.bounds.loopexit1
+; CHECK:        in.bounds:
+; CHECK-NEXT:     %addr = getelementptr i32, i32* %arr, i32 %idx
+; CHECK-NEXT:     store i32 0, i32* %addr
+; CHECK-NEXT:     %next = icmp slt i32 %idx, 100
+; CHECK-NEXT:     [[COND3:%[^ ]+]] = icmp slt i32 %idx, %exit.mainloop.at
+; CHECK-NEXT:     br i1 [[COND3]], label %loop, label %main.exit.selector
+; CHECK:        main.exit.selector:
+; CHECK-NEXT:     %idx.lcssa = phi i32 [ %idx, %in.bounds ]
+; CHECK-NEXT:     [[COND4:%[^ ]+]] = icmp slt i32 %idx.lcssa, 100
+; CHECK-NEXT:     br i1 [[COND4]], label %main.pseudo.exit, label %exit
+; CHECK-NOT: loop.preloop:
+; CHECK:        loop.postloop:
+; CHECK-NEXT:    %idx.postloop = phi i32 [ %idx.copy, %postloop ], [ %idx.next.postloop, %in.bounds.postloop ]
+; CHECK-NEXT:     %idx.next.postloop = add nuw nsw i32 %idx.postloop, 1
+; CHECK-NEXT:     %abc.postloop = icmp slt i32 %idx.postloop, %exit.mainloop.at
+; CHECK-NEXT:     br i1 %abc.postloop, label %in.bounds.postloop, label %out.of.bounds.loopexit
+
+entry:
+  %len = load i32, i32* %a_len_ptr, !range !0
+  br label %loop
+
+loop:
+  %idx = phi i32 [ 0, %entry ], [ %idx.next, %in.bounds ]
+  %idx.next = add nsw nuw i32 %idx, 1
+  %abc = icmp slt i32 %idx, %len
+  br i1 %abc, label %in.bounds, label %out.of.bounds
+
+in.bounds:
+  %addr = getelementptr i32, i32* %arr, i32 %idx
+  store i32 0, i32* %addr
+  %next = icmp slt i32 %idx, 100
+  br i1 %next, label %loop, label %exit
+
+out.of.bounds:
+  ret void
+
+exit:
+  ret void
+}
+
+; ULT condition for increasing loop from 0 to 100.
+define void @test_02(i32* %arr, i32* %a_len_ptr) #0 {
+
+; CHECK:      test_02
+; CHECK:        entry:
+; CHECK-NEXT:     %exit.mainloop.at = load i32, i32* %a_len_ptr, !range !0
+; CHECK-NEXT:     [[COND2:%[^ ]+]] = icmp ult i32 0, %exit.mainloop.at
+; CHECK-NEXT:     br i1 [[COND2]], label %loop.preheader, label %main.pseudo.exit
+; CHECK:        loop:
+; CHECK-NEXT:     %idx = phi i32 [ %idx.next, %in.bounds ], [ 0, %loop.preheader ]
+; CHECK-NEXT:     %idx.next = add nuw nsw i32 %idx, 1
+; CHECK-NEXT:     %abc = icmp ult i32 %idx, %exit.mainloop.at
+; CHECK-NEXT:     br i1 true, label %in.bounds, label %out.of.bounds.loopexit1
+; CHECK:        in.bounds:
+; CHECK-NEXT:     %addr = getelementptr i32, i32* %arr, i32 %idx
+; CHECK-NEXT:     store i32 0, i32* %addr
+; CHECK-NEXT:     %next = icmp ult i32 %idx, 100
+; CHECK-NEXT:     [[COND3:%[^ ]+]] = icmp ult i32 %idx, %exit.mainloop.at
+; CHECK-NEXT:     br i1 [[COND3]], label %loop, label %main.exit.selector
+; CHECK:        main.exit.selector:
+; CHECK-NEXT:     %idx.lcssa = phi i32 [ %idx, %in.bounds ]
+; CHECK-NEXT:     [[COND4:%[^ ]+]] = icmp ult i32 %idx.lcssa, 100
+; CHECK-NEXT:     br i1 [[COND4]], label %main.pseudo.exit, label %exit
+; CHECK-NOT: loop.preloop:
+; CHECK:        loop.postloop:
+; CHECK-NEXT:    %idx.postloop = phi i32 [ %idx.copy, %postloop ], [ %idx.next.postloop, %in.bounds.postloop ]
+; CHECK-NEXT:     %idx.next.postloop = add nuw nsw i32 %idx.postloop, 1
+; CHECK-NEXT:     %abc.postloop = icmp ult i32 %idx.postloop, %exit.mainloop.at
+; CHECK-NEXT:     br i1 %abc.postloop, label %in.bounds.postloop, label %out.of.bounds.loopexit
+
+entry:
+  %len = load i32, i32* %a_len_ptr, !range !0
+  br label %loop
+
+loop:
+  %idx = phi i32 [ 0, %entry ], [ %idx.next, %in.bounds ]
+  %idx.next = add nsw nuw i32 %idx, 1
+  %abc = icmp ult i32 %idx, %len
+  br i1 %abc, label %in.bounds, label %out.of.bounds
+
+in.bounds:
+  %addr = getelementptr i32, i32* %arr, i32 %idx
+  store i32 0, i32* %addr
+  %next = icmp ult i32 %idx, 100
+  br i1 %next, label %loop, label %exit
+
+out.of.bounds:
+  ret void
+
+exit:
+  ret void
+}
+
+; Same as test_01, but comparison happens against IV extended to a wider type.
+; This test ensures that IRCE rejects it and does not falsely assume that it was
+; a comparison against iv.next.
+; TODO: We can actually extend the recognition to cover this case.
+define void @test_03(i32* %arr, i64* %a_len_ptr) #0 {
+
+; CHECK:      test_03
+
+entry:
+  %len = load i64, i64* %a_len_ptr, !range !1
+  br label %loop
+
+loop:
+  %idx = phi i32 [ 0, %entry ], [ %idx.next, %in.bounds ]
+  %idx.next = add nsw nuw i32 %idx, 1
+  %idx.ext = sext i32 %idx to i64
+  %abc = icmp slt i64 %idx.ext, %len
+  br i1 %abc, label %in.bounds, label %out.of.bounds
+
+in.bounds:
+  %addr = getelementptr i32, i32* %arr, i32 %idx
+  store i32 0, i32* %addr
+  %next = icmp slt i32 %idx, 100
+  br i1 %next, label %loop, label %exit
+
+out.of.bounds:
+  ret void
+
+exit:
+  ret void
+}
+
+; Same as test_02, but comparison happens against IV extended to a wider type.
+; This test ensures that IRCE rejects it and does not falsely assume that it was
+; a comparison against iv.next.
+; TODO: We can actually extend the recognition to cover this case.
+define void @test_04(i32* %arr, i64* %a_len_ptr) #0 {
+
+; CHECK:      test_04
+
+entry:
+  %len = load i64, i64* %a_len_ptr, !range !1
+  br label %loop
+
+loop:
+  %idx = phi i32 [ 0, %entry ], [ %idx.next, %in.bounds ]
+  %idx.next = add nsw nuw i32 %idx, 1
+  %idx.ext = sext i32 %idx to i64
+  %abc = icmp ult i64 %idx.ext, %len
+  br i1 %abc, label %in.bounds, label %out.of.bounds
+
+in.bounds:
+  %addr = getelementptr i32, i32* %arr, i32 %idx
+  store i32 0, i32* %addr
+  %next = icmp ult i32 %idx, 100
+  br i1 %next, label %loop, label %exit
+
+out.of.bounds:
+  ret void
+
+exit:
+  ret void
+}
+
+!0 = !{i32 0, i32 50}
+!1 = !{i64 0, i64 50}