Fix out-of-order stepping behavior in programs with hoisted constants.

When the Constant Hoisting pass moves expensive constants into a
common block, it would assign a debug location equal to the last use
of that constant. While this is certainly intuitive, it places the
constant in an out-of-order location, according to the debug location
information. This produces out-of-order stepping when debugging
programs affected by this pass.

This patch creates in-order stepping behavior by merging the debug
locations for hoisted constants, and the new insertion point.

Patch by Matthew Voss!

Differential Revision: https://reviews.llvm.org/D38088

llvm-svn: 317827

1 files changed, 6 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp b/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp
index 58a910ded21..e4b08c5ed30 100644
--- a/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp
@@ -60,6 +60,7 @@
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Scalar.h"
 #include "llvm/Transforms/Utils/Local.h"
+#include "llvm/IR/DebugInfoMetadata.h"
 #include <algorithm>
 #include <cassert>
 #include <cstdint>
@@ -716,6 +717,9 @@ bool ConstantHoistingPass::emitBaseConstants() {
       IntegerType *Ty = ConstInfo.BaseConstant->getType();
       Instruction *Base =
           new BitCastInst(ConstInfo.BaseConstant, Ty, "const", IP);
+
+      Base->setDebugLoc(IP->getDebugLoc());
+
       DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseConstant
                    << ") to BB " << IP->getParent()->getName() << '\n'
                    << *Base << '\n');
@@ -734,6 +738,8 @@ bool ConstantHoistingPass::emitBaseConstants() {
             emitBaseConstants(Base, RCI.Offset, U);
             ReBasesNum++;
           }
+
+          Base->setDebugLoc(DILocation::getMergedLocation(Base->getDebugLoc(), U.Inst->getDebugLoc()));
         }
       }
       UsesNum = Uses;
@@ -742,7 +748,6 @@ bool ConstantHoistingPass::emitBaseConstants() {
       assert(!Base->use_empty() && "The use list is empty!?");
       assert(isa<Instruction>(Base->user_back()) &&
              "All uses should be instructions.");
-      Base->setDebugLoc(cast<Instruction>(Base->user_back())->getDebugLoc());
     }
     (void)UsesNum;
     (void)ReBasesNum;