[PATCH] D27857: [PATCH][DWARF] Don't propagate or set debug locations for PRE loads and associated address calculations

[Pieb, Wolfgang via llvm-commits]

Does this only apply to cases that would be a problem for profile accuracy - or does it apply in cases where code is moved within a single basic block? (not sure which kind of moves GVN/PRE do nor which ones your change is proposing to change the debug info for)

These are only cases where instructions are moved to or materialized in different basic blocks, so they could potentially affect profile accuracy.

-- wolfgang


The modified test case phi-translate.ll covers all cases.


https://reviews.llvm.org/D27857

Files:
  lib/Transforms/Scalar/GVN.cpp
  test/Transforms/GVN/PRE/phi-translate.ll


Index: test/Transforms/GVN/PRE/phi-translate.ll
===================================================================
--- test/Transforms/GVN/PRE/phi-translate.ll
+++ test/Transforms/GVN/PRE/phi-translate.ll
@@ -4,18 +4,17 @@

 ; CHECK-LABEL: @foo(
 ; CHECK: entry.end_crit_edge:
-; CHECK:   %j.phi.trans.insert = sext i32 %x to i64, !dbg [[J_LOC:![0-9]+]]
-; CHECK:   %q.phi.trans.insert = getelementptr {{.*}}, !dbg [[Q_LOC:![0-9]+]]
-; CHECK:   %n.pre = load i32, i32* %q.phi.trans.insert, !dbg [[N_LOC:![0-9]+]]
+; CHECK: %[[INDEX:[a-z0-9.]+]] = sext i32 %x to i64{{$}}
+; CHECK: %[[ADDRESS:[a-z0-9.]+]] = getelementptr [100 x i32], [100 x i32]* @G, i64 0, i64 %[[INDEX]]{{$}}
+; CHECK:   %n.pre = load i32, i32* %[[ADDRESS]]{{$}}
+; CHECK: br label %end
 ; CHECK: then:
 ; CHECK:   store i32 %z
 ; CHECK: end:
-; CHECK:   %n = phi i32 [ %n.pre, %entry.end_crit_edge ], [ %z, %then ], !dbg [[N_LOC]]
+; CHECK:   %n = phi i32 [ %n.pre, %entry.end_crit_edge ], [ %z, %then ], !dbg [[N_LOC:![0-9]+]]
 ; CHECK:   ret i32 %n

-; CHECK-DAG: [[J_LOC]] = !DILocation(line: 45, column: 1, scope: !{{.*}})
-; CHECK-DAG: [[Q_LOC]] = !DILocation(line: 46, column: 1, scope: !{{.*}})
-; CHECK-DAG: [[N_LOC]] = !DILocation(line: 47, column: 1, scope: !{{.*}})
+; CHECK: [[N_LOC]] = !DILocation(line: 47, column: 1, scope: !{{.*}})

 @G = external global [100 x i32]
 define i32 @foo(i32 %x, i32 %z) !dbg !6 {
Index: lib/Transforms/Scalar/GVN.cpp
===================================================================
--- lib/Transforms/Scalar/GVN.cpp
+++ lib/Transforms/Scalar/GVN.cpp
@@ -1572,6 +1572,13 @@

   // Assign value numbers to the new instructions.
   for (Instruction *I : NewInsts) {
+    // Instructions that have been inserted in predecessor(s) to materialize
+    // the load address do not retain their original debug locations. Doing
+    // so could lead to erratic source attributions.
+    // FIXME: How do we retain source locations without causing poor debugging
+    // behavior?
+    I->setDebugLoc(DebugLoc());
+
     // FIXME: We really _ought_ to insert these value numbers into their
     // parent's availability map.  However, in doing so, we risk getting into
     // ordering issues.  If a block hasn't been processed yet, we would be
@@ -1601,8 +1608,10 @@
     if (auto *RangeMD = LI->getMetadata(LLVMContext::MD_range))
       NewLoad->setMetadata(LLVMContext::MD_range, RangeMD);

-    // Transfer DebugLoc.
-    NewLoad->setDebugLoc(LI->getDebugLoc());
+    // We do not propagate the old load's debug location, because the new
+    // load now lives in a different BB.
+    // FIXME: How do we retain source locations without causing poor debugging
+    // behavior?

     // Add the newly created load.
     ValuesPerBlock.push_back(AvailableValueInBlock::get(UnavailablePred,

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