IR printing improvement for loop passes

[fedor.sergeev]

Summary

Loop-pass printing is somewhat deficient since it does not provide the
context around the loop (e.g. preheader). This context information becomes
pretty essential when analyzing transformations that move stuff out of the loop.

Extending printLoop to cover preheader and exit blocks (if any).

[sanjoy]

lgtm

lib/Analysis/LoopInfo.cpp

 PreservedAnalyses LoopPrinterPass::run(Function &F,
                                        FunctionAnalysisManager &AM) {
   AM.getResult<LoopAnalysis>(F).print(OS);
   return PreservedAnalyses::all();
 }
 
 void llvm::printLoop(Loop &L, raw_ostream &OS, const std::string &Banner) {
   OS << Banner;
+
+  auto *PreHeader = L.getLoopPreheader();
+  if (PreHeader) {
+    OS << "\n; Preheader:";
+    PreHeader->print(OS);
+    OS << "\n; Loop:";
+  }
+
   for (auto *Block : L.blocks())
     if (Block)
       Block->print(OS);
     else
       OS << "Printing <null> block";
+
+  SmallVector<BasicBlock *, 8> ExitBlocks;
+  L.getExitBlocks(ExitBlocks);
+  if (!ExitBlocks.empty()) {
+    OS << "\n; Exit blocks";
+    for (auto *Block : ExitBlocks)
+      if (Block)
+        Block->print(OS);
+      else
+        OS << "Printing <null> block";
+  }
 }
 
 //===----------------------------------------------------------------------===//
 // LoopInfo implementation
 //
 
 char LoopInfoWrapperPass::ID = 0;
 INITIALIZE_PASS_BEGIN(LoopInfoWrapperPass, "loops", "Natural Loop Information",

test/Other/loop-pass-printer.ll

+; This test checks -print-after/before on loop passes
+; Besides of the loop itself it should be dumping loop pre-header and exits.
+;
+; RUN: opt < %s 2>&1 -disable-output \
+; RUN:     -loop-deletion -print-before=loop-deletion \
+; RUN:     | FileCheck %s -check-prefix=DEL
+; RUN: opt < %s 2>&1 -disable-output \
+; RUN:     -loop-unroll -print-after=loop-unroll -filter-print-funcs=bar \
+; RUN:     | FileCheck %s -check-prefix=BAR
+;
+
+; DEL:      IR Dump Before
+; DEL-SAME: dead loops
+; DEL:      ; Preheader:
+; DEL-NEXT:  %idx = alloca i32, align 4
+; DEL:      ; Loop:
+; DEL-NEXT:  loop:
+; DEL:       cont:
+; DEL:      ; Exit blocks
+; DEL:       done:
+; DEL:      IR Dump Before
+; DEL-SAME: dead loops
+; DEL:      ; Preheader:
+; DEL-NEXT:  br label %loop
+; DEL:      ; Loop:
+; DEL-NEXT:  loop:
+; DEL:      ; Exit blocks
+; DEL:       end:
+
+; BAR:      IR Dump After
+; BAR-SAME: Unroll
+; BAR:      ; Preheader:
+; BAR-NEXT:  br label %loop
+; BAR:      ; Loop:
+; BAR-NEXT:  loop:
+; BAR:      ; Exit blocks
+; BAR:       end:
+; BAR-NOT: IR Dump Before
+; BAR-NOT:  ; Loop
+
+define void @foo(){
+  %idx = alloca i32, align 4
+  store i32 0, i32* %idx, align 4
+  br label %loop
+
+loop:
+  %1 = load i32, i32* %idx, align 4
+  %2 = icmp slt i32 %1, 10
+  br i1 %2, label %cont, label %done
+
+cont:
+  %3 = load i32, i32* %idx, align 4
+  %4 = add nsw i32 %3, 1
+  store i32 %4, i32* %idx, align 4
+  br label %loop
+
+done:
+  ret void
+}
+
+define void @bar(){
+  br label %loop
+loop:
+  br i1 1, label %loop, label %end
+end:
+  ret void
+}