[llvm-commits] [llvm] r154867 - in /llvm/trunk: lib/CodeGen/MachineBasicBlock.cpp test/CodeGen/Thumb2/thumb2-ifcvt2.ll test/CodeGen/X86/br-fold.ll

[Chandler Carruth]

Author: chandlerc
Date: Mon Apr 16 17:03:00 2012
New Revision: 154867

URL: http://llvm.org/viewvc/llvm-project?rev=154867&view=rev
Log:
Fix updateTerminator to be resiliant to degenerate terminators where
both fallthrough and a conditional branch target the same successor.
Gracefully delete the conditional branch and introduce any unconditional
branch needed to reach the actual successor. This fixes memory
corruption in 2009-06-15-RegScavengerAssert.ll and possibly other tests.

Also, while I'm here fix a latent bug I spotted by inspection. I never
applied the same fundamental fix to this fallthrough successor finding
logic that I did to the logic used when there are no conditional
branches. As a consequence it would have selected landing pads had they
be aligned in just the right way here. I don't have a test case as
I spotted this by inspection, and the previous time I found this
required have of TableGen's source code to produce it. =/ I hate backend
bugs. ;]

Thanks to Jim Grosbach for helping me reason through this and reviewing
the fix.

Modified:
    llvm/trunk/lib/CodeGen/MachineBasicBlock.cpp
    llvm/trunk/test/CodeGen/Thumb2/thumb2-ifcvt2.ll
    llvm/trunk/test/CodeGen/X86/br-fold.ll

Modified: llvm/trunk/lib/CodeGen/MachineBasicBlock.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachineBasicBlock.cpp?rev=154867&r1=154866&r2=154867&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/MachineBasicBlock.cpp (original)
+++ llvm/trunk/lib/CodeGen/MachineBasicBlock.cpp Mon Apr 16 17:03:00 2012
@@ -392,22 +392,44 @@
         TII->InsertBranch(*this, TBB, 0, Cond, dl);
       }
     } else {
+      // Walk through the successors and find the successor which is not
+      // a landing pad and is not the conditional branch destination (in TBB)
+      // as the fallthrough successor.
+      MachineBasicBlock *FallthroughBB = 0;
+      for (succ_iterator SI = succ_begin(), SE = succ_end(); SI != SE; ++SI) {
+        if ((*SI)->isLandingPad() || *SI == TBB)
+          continue;
+        assert(!FallthroughBB && "Found more than one fallthrough successor.");
+        FallthroughBB = *SI;
+      }
+      if (!FallthroughBB && canFallThrough()) {
+        // We fallthrough to the same basic block as the conditional jump
+        // targets. Remove the conditional jump, leaving unconditional
+        // fallthrough.
+        // FIXME: This does not seem like a reasonable pattern to support, but it
+        // has been seen in the wild coming out of degenerate ARM test cases.
+        TII->RemoveBranch(*this);
+
+        // Finally update the unconditional successor to be reached via a branch
+        // if it would not be reached by fallthrough.
+        if (!isLayoutSuccessor(TBB))
+          TII->InsertBranch(*this, TBB, 0, Cond, dl);
+        return;
+      }
+
       // The block has a fallthrough conditional branch.
-      MachineBasicBlock *MBBA = *succ_begin();
-      MachineBasicBlock *MBBB = *llvm::next(succ_begin());
-      if (MBBA == TBB) std::swap(MBBB, MBBA);
       if (isLayoutSuccessor(TBB)) {
         if (TII->ReverseBranchCondition(Cond)) {
           // We can't reverse the condition, add an unconditional branch.
           Cond.clear();
-          TII->InsertBranch(*this, MBBA, 0, Cond, dl);
+          TII->InsertBranch(*this, FallthroughBB, 0, Cond, dl);
           return;
         }
         TII->RemoveBranch(*this);
-        TII->InsertBranch(*this, MBBA, 0, Cond, dl);
-      } else if (!isLayoutSuccessor(MBBA)) {
+        TII->InsertBranch(*this, FallthroughBB, 0, Cond, dl);
+      } else if (!isLayoutSuccessor(FallthroughBB)) {
         TII->RemoveBranch(*this);
-        TII->InsertBranch(*this, TBB, MBBA, Cond, dl);
+        TII->InsertBranch(*this, TBB, FallthroughBB, Cond, dl);
       }
     }
   }

Modified: llvm/trunk/test/CodeGen/Thumb2/thumb2-ifcvt2.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/Thumb2/thumb2-ifcvt2.ll?rev=154867&r1=154866&r2=154867&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/Thumb2/thumb2-ifcvt2.ll (original)
+++ llvm/trunk/test/CodeGen/Thumb2/thumb2-ifcvt2.ll Mon Apr 16 17:03:00 2012
@@ -83,7 +83,7 @@
 entry:
 ; CHECK: t2:
 ; CHECK: cmp r0, #0
-; CHECK: bne
+; CHECK: %growMapping.exit
 	br i1 undef, label %bb.i.i3, label %growMapping.exit
 
 bb.i.i3:		; preds = %entry

Modified: llvm/trunk/test/CodeGen/X86/br-fold.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/br-fold.ll?rev=154867&r1=154866&r2=154867&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/br-fold.ll (original)
+++ llvm/trunk/test/CodeGen/X86/br-fold.ll Mon Apr 16 17:03:00 2012
@@ -1,7 +1,7 @@
 ; RUN: llc -march=x86-64 < %s | FileCheck %s
 
 ; CHECK: orq
-; CHECK-NEXT: je
+; CHECK-NEXT: .LBB0_1
 
 @_ZN11xercesc_2_513SchemaSymbols21fgURI_SCHEMAFORSCHEMAE = external constant [33 x i16], align 32 ; <[33 x i16]*> [#uses=1]
 @_ZN11xercesc_2_56XMLUni16fgNotationStringE = external constant [9 x i16], align 16 ; <[9 x i16]*> [#uses=1]