[PATCH] D35979: [X86] Fix a crash in FEntryInserter Pass.

[Manoj Gupta via Phabricator via llvm-commits]

manojgupta created this revision.

FEntryInserter pass unconditionally derefs the first Instruction
in the first Basic Block. The pass crashes when the first
BasicBlock is empty. Fix the crash by not dereferencing the basic
Block iterator. This fixes an issue observed when building Linux kernel
4.4 with clang.

Fixes PR33971.


https://reviews.llvm.org/D35979

Files:
  lib/CodeGen/FEntryInserter.cpp
  test/CodeGen/X86/fentry-insertion.ll


Index: test/CodeGen/X86/fentry-insertion.ll
===================================================================
--- test/CodeGen/X86/fentry-insertion.ll
+++ test/CodeGen/X86/fentry-insertion.ll
@@ -12,5 +12,21 @@
 ; CHECK: retq
 }
 
-attributes #0 = { "fentry-call"="true" }
+; Function Attrs: inlinehint minsize noredzone nounwind optsize sspstrong
+define void @test2() #1 {
+entry:
+  br label %bb1
+bb1:
+  call void @address_taken(i64 ptrtoint (i8* blockaddress(@test2, %bb1) to i64), i32 512)
+  ret void
 
+; CHECK-LABEL: @test2
+; CHECK: callq __fentry__
+; CHECK-NOT: mcount
+; CHECK: retq
+}
+
+; Function Attrs: minsize noredzone optsize
+declare void @address_taken(i64, i32) local_unnamed_addr
+attributes #0 = { "fentry-call"="true" }
+attributes #1 = { inlinehint minsize noredzone nounwind optsize sspstrong "fentry-call"="true" }
Index: lib/CodeGen/FEntryInserter.cpp
===================================================================
--- lib/CodeGen/FEntryInserter.cpp
+++ lib/CodeGen/FEntryInserter.cpp
@@ -41,10 +41,9 @@
     return false;
 
   auto &FirstMBB = *MF.begin();
-  auto &FirstMI = *FirstMBB.begin();
 
   auto *TII = MF.getSubtarget().getInstrInfo();
-  BuildMI(FirstMBB, FirstMI, FirstMI.getDebugLoc(),
+  BuildMI(FirstMBB, FirstMBB.begin(), DebugLoc(),
           TII->get(TargetOpcode::FENTRY_CALL));
   return true;
 }


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