[CodegenPrepare] Guard against degenerate branches

[vchuravy]

Summary

Guard against a potential crash observed in https://github.com/JuliaLang/julia/issues/32994#issuecomment-524249628
If two branches are collapsed we can encounter a degenerate conditional branch TBB==FBB.
The subsequent code assumes that they differ, so we exit out early.

[spatel]

llvm/lib/CodeGen/CodeGenPrepare.cpp

7349–7351

Formatting is off, and how does that work in a debug build? No braces for the 'if'. Might be better to just leave out the LLVM_DEBUG line.

llvm/test/CodeGen/X86/codegen-prepare-collapse.ll

2

Is there some way to create a test with 'opt -codegenprepare -S' rather than 'llc'? That would be preferable since the bug is not x86-specific.
If llc is the only way, then we still probably should produce some CHECK lines for FileCheck by using "utils/update_llc_test_checks.py"

[vchuravy]

• update aaccording to comments

[vchuravy]

Sorry that it took so long for me to finish this.

llvm/test/CodeGen/X86/codegen-prepare-collapse.ll

2

I haven't been able to reproduce the crash with opt, if I recall correctly it needs a fully instantiated TargetMachine to trigger the crash

[merge_guards_bot]

Unit tests: fail. 57754 tests passed, 1 failed and 719 were skipped.

failed: Clang.Driver/check-time-trace-sections.cpp

clang-format: pass.

Build artifacts: console-log.txt, CMakeCache.txt, test-results.xml, diff.json

[spatel]

LGTM

llvm/lib/CodeGen/CodeGenPrepare.cpp

7348

Nit: Add period at end of comment sentence.

llvm/test/CodeGen/X86/codegen-prepare-collapse.ll

15

typo: occure -> occur

llvm/lib/CodeGen/CodeGenPrepare.cpp

for (auto &BB : F) {
     BasicBlock *TBB, *FBB;
     if (!match(BB.getTerminator(), m_Br(m_OneUse(m_BinOp(LogicOp)), TBB, FBB)))
       continue;
 
     auto *Br1 = cast<BranchInst>(BB.getTerminator());
     if (Br1->getMetadata(LLVMContext::MD_unpredictable))
       continue;
 
+    // The merging of mostly empty BB can cause a degenerate branch.
+    if (TBB == FBB)
+      continue;
+
     unsigned Opc;
     Value *Cond1, *Cond2;
     if (match(LogicOp, m_And(m_OneUse(m_Value(Cond1)),
                              m_OneUse(m_Value(Cond2)))))
       Opc = Instruction::And;
     else if (match(LogicOp, m_Or(m_OneUse(m_Value(Cond1)),
                                  m_OneUse(m_Value(Cond2)))))
       Opc = Instruction::Or;

llvm/test/CodeGen/X86/codegen-prepare-collapse.ll

+; RUN: llc -fast-isel=true -O1 -mtriple=x86_64-unkown-linux-gnu -start-before=codegenprepare -stop-after=codegenprepare -o - < %s | FileCheck %s
+
+; CHECK-LABEL: @foo
+define void @foo() {
+top:
+; CHECK: br label %L34
+  br label %L34
+
+L34:                                              ; preds = %L34, %L34, %top
+  %.sroa.075.0 = phi i64 [ undef, %top ], [ undef, %L34 ], [ undef, %L34 ]
+  %0 = icmp sgt i8 undef, -1
+  %cond5896 = icmp eq i8 0, 2
+  %cond58 = and i1 %cond5896, %0
+; During codegenprepare such degenerate branches can occur and should not
+; lead to crashes.
+; CHECK: br label %L34
+  br i1 %cond58, label %L34, label %L34
+}