[llvm] r315619 - [DWARF] Fix bad comparator in sortGlobalExprs.

[Eli Friedman via llvm-commits]

Author: efriedma
Date: Thu Oct 12 13:54:08 2017
New Revision: 315619

URL: http://llvm.org/viewvc/llvm-project?rev=315619&view=rev
Log:
[DWARF] Fix bad comparator in sortGlobalExprs.

The comparator passed to std::sort must provide a strict weak ordering;
otherwise, the behavior is undefined.

Fixes an assertion failure generating debug info for globals
split by GlobalOpt. I have a testcase, but not sure how to reduce it,
so not included here.  (Someone else came up with a testcase, but I
can't reproduce the crash with it, presumably because my version of LLVM
ends up sorting the array differently.)

This isn't really a complete fix (see the FIXME in the patch), but at
least it doesn't have undefined behavior.

Differential Revision: https://reviews.llvm.org/D38830


Modified:
    llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

Modified: llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp?rev=315619&r1=315618&r2=315619&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp (original)
+++ llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp Thu Oct 12 13:54:08 2017
@@ -543,13 +543,18 @@ static SmallVectorImpl<DwarfCompileUnit:
 sortGlobalExprs(SmallVectorImpl<DwarfCompileUnit::GlobalExpr> &GVEs) {
   std::sort(GVEs.begin(), GVEs.end(),
             [](DwarfCompileUnit::GlobalExpr A, DwarfCompileUnit::GlobalExpr B) {
-              if (A.Expr != B.Expr && A.Expr && B.Expr) {
-                auto FragmentA = A.Expr->getFragmentInfo();
-                auto FragmentB = B.Expr->getFragmentInfo();
-                if (FragmentA && FragmentB)
-                  return FragmentA->OffsetInBits < FragmentB->OffsetInBits;
-              }
-              return false;
+              // Sort order: first null exprs, then exprs without fragment
+              // info, then sort by fragment offset in bits.
+              // FIXME: Come up with a more comprehensive comparator so
+              // the sorting isn't non-deterministic, and so the following
+              // std::unique call works correctly.
+              if (!A.Expr || !B.Expr)
+                return !!B.Expr;
+              auto FragmentA = A.Expr->getFragmentInfo();
+              auto FragmentB = B.Expr->getFragmentInfo();
+              if (!FragmentA || !FragmentB)
+                return !!FragmentB;
+              return FragmentA->OffsetInBits < FragmentB->OffsetInBits;
             });
   GVEs.erase(std::unique(GVEs.begin(), GVEs.end(),
                          [](DwarfCompileUnit::GlobalExpr A,