[PATCH] D31558: CodeGen: BranchFolding: Merge identical blocks, even if they are short.

[Kyle Butt via Phabricator via llvm-commits]

iteratee updated this revision to Diff 94123.
iteratee edited the summary of this revision.
iteratee added a comment.

Now merge in a far more limited case, only if one of the blocks is missing a fallthrough in or out.
Also, only after layout so that we reliably know about fallthrough.


https://reviews.llvm.org/D31558

Files:
  lib/CodeGen/BranchFolding.cpp
  test/CodeGen/X86/tail-merge-identical.ll


Index: test/CodeGen/X86/tail-merge-identical.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/tail-merge-identical.ll
@@ -0,0 +1,41 @@
+; RUN: llc -o - -verify-machineinstrs %s | FileCheck %s
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+ at data = external global [3 x i32], align 4
+ at store = external global i32, align 4
+
+; %else1 and %then2 end up lowering to identical blocks. These blocks should be
+; merged during tail-merging.
+; CHECK-LABEL: merge_identical_blocks
+; CHECK: movl $data+4
+; CHECK-NOT: movl $data+4
+; CHECK: retq
+define void @merge_identical_blocks(i1 %a, i1 %b) {
+entry:
+  br label %if1
+
+if1:                                              ; predfs = %entry
+  br i1 %a, label %else1, label %if2
+
+else1:                                            ; preds = %if1
+  %ptr.else1 = getelementptr inbounds [3 x i32], [3 x i32]* @data, i64 0, i32 1
+  br label %phi_join
+
+if2:                                              ; preds = %if1
+  br i1 %b, label %then2, label %else2
+
+then2:                                            ; preds = %if2
+  %ptr.then2 = getelementptr inbounds [3 x i32], [3 x i32]* @data, i64 0, i32 1
+  br label %phi_join
+
+else2:                                            ; preds = %if2
+  %ptr.else2 = getelementptr inbounds [3 x i32], [3 x i32]* @data, i64 0, i32 2
+  br label %phi_join
+
+phi_join:                                         ; preds = %else1, %then2, %else2
+  %val.ptr = phi i32* [ %ptr.else1, %else1 ], [ %ptr.then2, %then2 ], [ %ptr.else2, %else2 ]
+  %val = load i32, i32* %val.ptr, align 4
+  store i32 %val, i32* @store, align 4
+  ret void
+}
Index: lib/CodeGen/BranchFolding.cpp
===================================================================
--- lib/CodeGen/BranchFolding.cpp
+++ lib/CodeGen/BranchFolding.cpp
@@ -600,6 +600,22 @@
   if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
     return true;
 
+  // If both blocks are identical and end in a branch, merge them unless they
+  // both have a fallthrough predecessor and successor.
+  // We can only do this after block placement because it depends on whether
+  // there are fallthroughs, and we don't know until after layout.
+  if (AfterPlacement && I1 == MBB1->begin() && I2 == MBB2->begin()) {
+    auto BothFallThrough = [](MachineBasicBlock *MBB) {
+      if (MBB->succ_size() != 0 && !MBB->canFallThrough())
+        return false;
+      MachineFunction::iterator I(MBB);
+      MachineFunction *MF = MBB->getParent();
+      return (MBB != &*MF->begin()) && std::prev(I)->canFallThrough();
+    };
+    if (!BothFallThrough(MBB1) || !BothFallThrough(MBB2))
+      return true;
+  }
+
   // If both blocks have an unconditional branch temporarily stripped out,
   // count that as an additional common instruction for the following
   // heuristics. This heuristic is only accurate for single-succ blocks, so to


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D31558.94123.patch
Type: text/x-patch
Size: 2992 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20170404/78bfaf9a/attachment.bin>