[DWARF] Null out the debug locs of load instructions that have been moved by GVN

performing partial redundancy elimination (PRE). Not doing so can cause jumpy line
tables and confusing (though correct) source attributions.

Differential Revision: https://reviews.llvm.org/D27857

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291037 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Scalar/GVN.cpp b/lib/Transforms/Scalar/GVN.cpp
index 9485bfd7c296cd9cb3fba1df61553cc13967c10d..0137378b828b470c422c73543940c0b074733d4c 100644 (file)
--- a/lib/Transforms/Scalar/GVN.cpp
+++ b/lib/Transforms/Scalar/GVN.cpp
@@ -1572,6 +1572,13 @@ bool GVN::PerformLoadPRE(LoadInst *LI, AvailValInBlkVect &ValuesPerBlock,
 
   // 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 confusing (but correct) 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 +1608,11 @@ bool GVN::PerformLoadPRE(LoadInst *LI, AvailValInBlkVect &ValuesPerBlock,
     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, and we want to avoid a jumpy line
+    // table.
+    // FIXME: How do we retain source locations without causing poor debugging
+    // behavior?
 
     // Add the newly created load.
     ValuesPerBlock.push_back(AvailableValueInBlock::get(UnavailablePred,

diff --git a/test/Transforms/GVN/PRE/phi-translate.ll b/test/Transforms/GVN/PRE/phi-translate.ll
index 42335486910c7f9f39729bd8b11793f1917de372..1f6c7c8d33ea79d8ecebda974ffd89d12b8c2d8f 100644 (file)
--- a/test/Transforms/GVN/PRE/phi-translate.ll
+++ b/test/Transforms/GVN/PRE/phi-translate.ll
@@ -4,18 +4,17 @@ target datalayout = "e-p:64:64:64"
 
 ; 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 {