From: Hans Wennborg <hans@hanshq.net>
Date: Tue, 20 Aug 2019 09:45:09 +0000 (+0000)
Subject: Merging r369168:
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=5571053b73c4b3e0baa46f7c39825a8113d6521b;p=llvm

Merging r369168:
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r369168 | spatel | 2019-08-17 01:10:34 +0200 (Sat, 17 Aug 2019) | 16 lines

[CodeGenPrepare] Fix use-after-free

If OptimizeExtractBits() encountered a shift instruction with no operands at all,
it would erase the instruction, but still return false.

This previously didn’t matter because its caller would always return after
processing the instruction, but https://reviews.llvm.org/D63233 changed the
function’s caller to fall through if it returned false, which would then cause
a use-after-free detectable by ASAN.

This change makes OptimizeExtractBits return true if it removes a shift
instruction with no users, terminating processing of the instruction.

Patch by: @brentdax (Brent Royal-Gordon)

Differential Revision: https://reviews.llvm.org/D66330
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_90@369355 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/CodeGen/CodeGenPrepare.cpp b/lib/CodeGen/CodeGenPrepare.cpp
index 52b4bbea012..e6f2aa9ef93 100644
--- a/lib/CodeGen/CodeGenPrepare.cpp
+++ b/lib/CodeGen/CodeGenPrepare.cpp
@@ -1682,10 +1682,11 @@ static bool OptimizeExtractBits(BinaryOperator *ShiftI, ConstantInt *CI,
     TheUse = InsertedShift;
   }
 
-  // If we removed all uses, nuke the shift.
+  // If we removed all uses, or there are none, nuke the shift.
   if (ShiftI->use_empty()) {
     salvageDebugInfo(*ShiftI);
     ShiftI->eraseFromParent();
+    MadeChange = true;
   }
 
   return MadeChange;
diff --git a/test/Transforms/CodeGenPrepare/sink-shift-and-trunc.ll b/test/Transforms/CodeGenPrepare/sink-shift-and-trunc.ll
index 20a436edd10..609b6bf679a 100644
--- a/test/Transforms/CodeGenPrepare/sink-shift-and-trunc.ll
+++ b/test/Transforms/CodeGenPrepare/sink-shift-and-trunc.ll
@@ -58,6 +58,23 @@ return:                                           ; preds = %if.then17, %if.end1
   ret i32 %retval.0, !dbg !63
 }
 
+; CodeGenPrepare was erasing the unused lshr instruction, but then further
+; processing the instruction after it was freed. If this bug is still present,
+; this test will always crash in an LLVM built with ASAN enabled, and may
+; crash even if ASAN is not enabled.
+
+define i32 @shift_unused(i32 %a) {
+; CHECK-LABEL: @shift_unused(
+; CHECK-NEXT:  BB2:
+; CHECK-NEXT:    ret i32 [[A:%.*]]
+;
+  %as = lshr i32 %a, 3
+  br label %BB2
+
+BB2:
+  ret i32 %a
+}
+
 ; CHECK: [[shift1_loc]] = !DILocation(line: 1
 ; CHECK: [[trunc1_loc]] = !DILocation(line: 2
 ; CHECK: [[shift2_loc]] = !DILocation(line: 3