[X86] Disable conditional tail calls (PR31257)

They are currently modelled incorrectly (as calls, which clobber
registers, confusing e.g. Machine Copy Propagation).

Reverting until we figure out the proper solution.

llvm-svn: 294348

1 files changed, 0 insertions, 37 deletions

diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index c632eb7ab7d..b196bf98d27 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -49,7 +49,6 @@ STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
 STATISTIC(NumBranchOpts, "Number of branches optimized");
 STATISTIC(NumTailMerge , "Number of block tails merged");
 STATISTIC(NumHoist     , "Number of times common instructions are hoisted");
-STATISTIC(NumTailCalls,  "Number of tail calls optimized");
 
 static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
                               cl::init(cl::BOU_UNSET), cl::Hidden);
@@ -1394,42 +1393,6 @@ ReoptimizeBlock:
     }
   }
 
-  if (!IsEmptyBlock(MBB) && MBB->pred_size() == 1 &&
-      MF.getFunction()->optForSize()) {
-    // Changing "Jcc foo; foo: jmp bar;" into "Jcc bar;" might change the branch
-    // direction, thereby defeating careful block placement and regressing
-    // performance. Therefore, only consider this for optsize functions.
-    MachineInstr &TailCall = *MBB->getFirstNonDebugInstr();
-    if (TII->isUnconditionalTailCall(TailCall)) {
-      MachineBasicBlock *Pred = *MBB->pred_begin();
-      MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
-      SmallVector<MachineOperand, 4> PredCond;
-      bool PredAnalyzable =
-          !TII->analyzeBranch(*Pred, PredTBB, PredFBB, PredCond, true);
-
-      if (PredAnalyzable && !PredCond.empty() && PredTBB == MBB) {
-        // The predecessor has a conditional branch to this block which consists
-        // of only a tail call. Try to fold the tail call into the conditional
-        // branch.
-        if (TII->canMakeTailCallConditional(PredCond, TailCall)) {
-          // TODO: It would be nice if analyzeBranch() could provide a pointer
-          // to the branch insturction so replaceBranchWithTailCall() doesn't
-          // have to search for it.
-          TII->replaceBranchWithTailCall(*Pred, PredCond, TailCall);
-          ++NumTailCalls;
-          Pred->removeSuccessor(MBB);
-          MadeChange = true;
-          return MadeChange;
-        }
-      }
-      // If the predecessor is falling through to this block, we could reverse
-      // the branch condition and fold the tail call into that. However, after
-      // that we might have to re-arrange the CFG to fall through to the other
-      // block and there is a high risk of regressing code size rather than
-      // improving it.
-    }
-  }
-
   // Analyze the branch in the current block.
   MachineBasicBlock *CurTBB = nullptr, *CurFBB = nullptr;
   SmallVector<MachineOperand, 4> CurCond;