[PATCH] D77651: [IfConversion] Disallow TrueBB == FalseBB for valid diamonds

[Mikael Holmén via Phabricator via llvm-commits]

uabelho created this revision.
uabelho added reviewers: efriedma, kparzysz.
Herald added a subscriber: hiraditya.
uabelho edited the summary of this revision.
uabelho added a comment.

I have no idea if this is the best fix for
 https://bugs.llvm.org/show_bug.cgi?id=45302
but I saw that we've made a similar fix for triangles in 957e090ac9 and thought maybe this could be an easy way out in this case.

Found this when compiling for my out of tree backend with a non-standard pipeline, so in normal cases I suppose the code will be cleanup up before we reach ifconversion.


This fixes PR45302.

Previously the case

    BB1
    / \
   |   |
  TBB FBB
   |   |
    \ /
    BB2

was treated as a valid diamond also when TBB and FBB was the same basic
block. This then lead to a failed assertion in IfConvertDiamond.

Since TBB == FBB is quite a degenerated case of a diamond, we now
don't treat it as a valid diamond anymore, and thus we will avoid the
trouble of making IfConvertDiamond handle it correctly.


https://reviews.llvm.org/D77651

Files:
  llvm/lib/CodeGen/IfConversion.cpp
  llvm/test/CodeGen/ARM/ifcvt_diamondSameTrueFalse.mir


Index: llvm/test/CodeGen/ARM/ifcvt_diamondSameTrueFalse.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/ARM/ifcvt_diamondSameTrueFalse.mir
@@ -0,0 +1,40 @@
+# RUN: llc -mtriple=thumbv7-apple-ios -o - %s -run-pass if-converter -verify-machineinstrs | FileCheck %s
+
+# Don't treat bb.1 as a valid diamond since IfConverter::IfConvertDiamond can't
+# handle it and used to hit an assertion instead.
+
+--- |
+  define void @func() minsize {
+    ret void
+  }
+...
+---
+name:            func
+body:             |
+  bb.0:
+
+    tBcc %bb.3, 1, $cpsr
+
+  bb.1:
+
+    tBcc %bb.2, 1, $cpsr
+    tB %bb.2, 14, $noreg
+
+  bb.2:
+
+  bb.3:
+  successors:
+    tBX_RET 14, _
+...
+
+# CHECK-LABEL: bb.0:
+# CHECK:         tBcc %bb.3, 1
+
+# CHECK-LABEL: bb.1:
+# CHECK:         tBcc %bb.2, 1
+# CHECK-NEXT:    tB %bb.2, 14
+
+# CHECK-LABEL: bb.2:
+
+# CHECK-LABEL: bb.3:
+# CHECK:         tBX_RET 14
Index: llvm/lib/CodeGen/IfConversion.cpp
===================================================================
--- llvm/lib/CodeGen/IfConversion.cpp
+++ llvm/lib/CodeGen/IfConversion.cpp
@@ -972,6 +972,11 @@
       FalseBBI.IsBeingAnalyzed || FalseBBI.IsDone)
     return false;
 
+  // If the True and False BBs are equal we're dealing with a degenerate case
+  // that we don't treat as a diamond.
+  if (TrueBBI.BB == FalseBBI.BB)
+    return false;
+
   MachineBasicBlock *TT = TrueBBI.TrueBB;
   MachineBasicBlock *FT = FalseBBI.TrueBB;
 


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