Remove assert from MachineLoop::getLoopPredecessor()

According to the documentation method returns predecessor
if the given loop's header has exactly one unique predecessor
outside the loop. Otherwise return null.

In reality it asserts if there is no predecessor outside of
the loop.

The testcase has the loop where predecessors outside of the
loop were not identified as analyzeBranch() was unable to
process the mask branch and returned true. That is also not
correct to assert for the truly dead loops.

Differential Revision: https://reviews.llvm.org/D67634

llvm-svn: 372405

2 files changed, 92 insertions, 2 deletions

diff --git a/llvm/include/llvm/Analysis/LoopInfoImpl.h b/llvm/include/llvm/Analysis/LoopInfoImpl.h
index 4c33dac9e21..2f38dde7b3a 100644
--- a/llvm/include/llvm/Analysis/LoopInfoImpl.h
+++ b/llvm/include/llvm/Analysis/LoopInfoImpl.h
@@ -200,8 +200,6 @@ BlockT *LoopBase<BlockT, LoopT>::getLoopPredecessor() const {
     }
   }
 
-  // Make sure there is only one exit out of the preheader.
-  assert(Out && "Header of loop has no predecessors from outside loop?");
   return Out;
 }
 
diff --git a/llvm/test/CodeGen/AMDGPU/loop_header_nopred.mir b/llvm/test/CodeGen/AMDGPU/loop_header_nopred.mir
new file mode 100644
index 00000000000..a1f23777e30
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/loop_header_nopred.mir
@@ -0,0 +1,92 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -march=amdgcn -o - -run-pass=block-placement -mcpu=gfx1010 -mattr=-inst-fwd-prefetch-bug -verify-machineinstrs %s | FileCheck -check-prefix=GCN %s
+
+# Used to fail with
+# Assertion `Out && "Header of loop has no predecessors from outside loop?"
+
+---
+name:            loop_header_nopred
+body:             |
+  ; GCN-LABEL: name: loop_header_nopred
+  ; GCN: bb.0:
+  ; GCN:   successors: %bb.2(0x40000000), %bb.3(0x40000000)
+  ; GCN:   S_CBRANCH_VCCZ %bb.3, implicit $vcc
+  ; GCN:   S_BRANCH %bb.2
+  ; GCN: bb.6 (align 64):
+  ; GCN:   successors: %bb.7(0x04000000), %bb.1(0x7c000000)
+  ; GCN:   S_CBRANCH_VCCNZ %bb.7, implicit $vcc
+  ; GCN: bb.1:
+  ; GCN:   successors: %bb.2(0x40000000), %bb.3(0x40000000)
+  ; GCN:   S_CBRANCH_VCCNZ %bb.2, implicit $vcc
+  ; GCN: bb.3:
+  ; GCN:   successors: %bb.4(0x40000000), %bb.6(0x40000000)
+  ; GCN:   SI_MASK_BRANCH %bb.6, implicit $exec
+  ; GCN:   S_BRANCH %bb.4
+  ; GCN: bb.2 (align 64):
+  ; GCN:   successors: %bb.4(0x40000000), %bb.6(0x40000000)
+  ; GCN:   SI_MASK_BRANCH %bb.6, implicit $exec
+  ; GCN:   S_BRANCH %bb.4
+  ; GCN: bb.4:
+  ; GCN:   successors: %bb.5(0x04000000), %bb.4(0x7c000000)
+  ; GCN:   S_NOP 0
+  ; GCN:   S_NOP 0
+  ; GCN:   S_NOP 0
+  ; GCN:   S_NOP 0
+  ; GCN:   S_NOP 0
+  ; GCN:   S_NOP 0
+  ; GCN:   S_NOP 0
+  ; GCN:   S_NOP 0
+  ; GCN:   S_NOP 0
+  ; GCN:   S_NOP 0
+  ; GCN:   S_NOP 0
+  ; GCN:   S_CBRANCH_EXECZ %bb.4, implicit $exec
+  ; GCN: bb.5:
+  ; GCN:   successors: %bb.6(0x80000000)
+  ; GCN:   S_BRANCH %bb.6
+  ; GCN: bb.7:
+  ; GCN:   S_ENDPGM 0
+  bb.0:
+    successors: %bb.1(0x80000000)
+
+  bb.1:
+    successors: %bb.2(0x40000000), %bb.3(0x40000000)
+
+    S_CBRANCH_VCCZ %bb.3, implicit $vcc
+    S_BRANCH %bb.2
+
+  bb.2:
+    successors: %bb.3(0x80000000)
+
+  bb.3:
+    successors: %bb.4(0x40000000), %bb.6(0x40000000)
+
+    SI_MASK_BRANCH %bb.6, implicit $exec
+    S_BRANCH %bb.4
+
+  bb.4:
+    successors: %bb.5(0x04000000), %bb.4(0x7c000000)
+
+    S_NOP 0
+    S_NOP 0
+    S_NOP 0
+    S_NOP 0
+    S_NOP 0
+    S_NOP 0
+    S_NOP 0
+    S_NOP 0
+    S_NOP 0
+    S_NOP 0
+    S_NOP 0
+    S_CBRANCH_EXECZ %bb.4, implicit $exec
+
+  bb.5:
+    successors: %bb.6(0x80000000)
+
+  bb.6:
+    successors: %bb.7(0x04000000), %bb.1(0x7c000000)
+
+    S_CBRANCH_VCCZ %bb.1, implicit $vcc
+
+  bb.7:
+    S_ENDPGM 0
+...