commit e941e2c44a610bd6451177593a711251b96ca7d2
author buzbee <buzbee@google.com> Mon Dec 05 12:38:17 2011 -0800
committer buzbee <buzbee@google.com> Mon Dec 05 12:47:29 2011 -0800
tree a4f9e5de729562426fe051d47c24f2e2c7f9f9f8
parent aed4be94da51b4fbb54c728151f0daf11535f6ab

Fix basic block construction

There was a bug in the CFG building mechanism that tiggered when
a backwards branch re-entered the current basic block at a point
other than the head.  In that case, we'd split the original basic block
in two.  However, the head of the old block then became the head
of the first of a new two block pair - and we really needed to continue
processing with the second of the pair.

Change-Id: Ieb9fdd5176d86cd0959bb9e91db9ca06075d4f21

src/compiler/Frontend.cc

diff --git a/src/compiler/Frontend.cc b/src/compiler/Frontend.cc
index 89f63be..ecc281b 100644
--- a/src/compiler/Frontend.cc
+++ b/src/compiler/Frontend.cc
@@ -451,9 +451,10 @@
 }
 
 /* Process instructions with the kInstrCanBranch flag */
-STATIC void processCanBranch(CompilationUnit* cUnit, BasicBlock* curBlock,
-                             MIR* insn, int curOffset, int width, int flags,
-                             const u2* codePtr, const u2* codeEnd)
+STATIC BasicBlock* processCanBranch(CompilationUnit* cUnit,
+                                    BasicBlock* curBlock, MIR* insn,
+                                    int curOffset, int width, int flags,
+                                    const u2* codePtr, const u2* codeEnd)
 {
     int target = curOffset;
     switch (insn->dalvikInsn.opcode) {
@@ -482,11 +483,22 @@
             LOG(FATAL) << "Unexpected opcode(" << (int)insn->dalvikInsn.opcode
                 << ") with kInstrCanBranch set";
     }
+    /*
+     * Some ugliness here.  It is possible that findBlock will
+     * split the current block.  In that case, we need to operate
+     * on the 2nd half on the split pair.  It isn't directly obvious
+     * when this happens, so we infer.
+     */
+    DCHECK(curBlock->lastMIRInsn == insn);
     BasicBlock *takenBlock = findBlock(cUnit, target,
                                        /* split */
                                        true,
                                        /* create */
                                        true);
+     if (curBlock->lastMIRInsn != insn) {
+         DCHECK(takenBlock->lastMIRInsn == insn);
+         curBlock = curBlock->fallThrough;
+    }
     curBlock->taken = takenBlock;
     oatSetBit(takenBlock->predecessors, curBlock->id);
 
@@ -521,6 +533,7 @@
                       true);
         }
     }
+    return curBlock;
 }
 
 /* Process instructions with the kInstrCanSwitch flag */
@@ -791,8 +804,8 @@
         int flags = dexGetFlagsFromOpcode(insn->dalvikInsn.opcode);
 
         if (flags & kInstrCanBranch) {
-            processCanBranch(cUnit.get(), curBlock, insn, curOffset, width, flags,
-                             codePtr, codeEnd);
+            curBlock = processCanBranch(cUnit.get(), curBlock, insn, curOffset,
+                                        width, flags, codePtr, codeEnd);
         } else if (flags & kInstrCanReturn) {
             curBlock->fallThrough = exitBlock;
             oatSetBit(exitBlock->predecessors, curBlock->id);