Implementation of asm-goto support in LLVM

This patch accompanies the RFC posted here:
http://lists.llvm.org/pipermail/llvm-dev/2018-October/127239.html

This patch adds a new CallBr IR instruction to support asm-goto
inline assembly like gcc as used by the linux kernel. This
instruction is both a call instruction and a terminator
instruction with multiple successors. Only inline assembly
usage is supported today.

This also adds a new INLINEASM_BR opcode to SelectionDAG and
MachineIR to represent an INLINEASM block that is also
considered a terminator instruction.

There will likely be more bug fixes and optimizations to follow
this, but we felt it had reached a point where we would like to
switch to an incremental development model.

Patch by Craig Topper, Alexander Ivchenko, Mikhail Dvoretckii

Differential Revision: https://reviews.llvm.org/D53765

llvm-svn: 353563

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index 1e04f79..14f56279 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -655,6 +655,16 @@ bool CodeGenPrepare::isMergingEmptyBlockProfitable(BasicBlock *BB,
         BB->getSinglePredecessor()->getSingleSuccessor()))
     return false;
 
+  // Skip merging if the block's successor is also a successor to any callbr
+  // that leads to this block.
+  // FIXME: Is this really needed? Is this a correctness issue?
+  for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
+    if (auto *CBI = dyn_cast<CallBrInst>((*PI)->getTerminator()))
+      for (unsigned i = 0, e = CBI->getNumSuccessors(); i != e; ++i)
+        if (DestBB == CBI->getSuccessor(i))
+          return false;
+  }
+
   // Try to skip merging if the unique predecessor of BB is terminated by a
   // switch or indirect branch instruction, and BB is used as an incoming block
   // of PHIs in DestBB. In such case, merging BB and DestBB would cause ISel to