[WebAssembly] Fix debug locations for ExplicitLocals pass

This is a reworked version of the reverted patch: https://reviews.llvm.org/D112487
Note that
a) it doesn't need the test changes anymore, and
b) I checked at least locally it passes other.test_pthread_lsan_leak

Differential Revision: https://reviews.llvm.org/D113208

1 files changed, 8 insertions, 3 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp
index a933d1a..910a4e5 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp
@@ -379,9 +379,14 @@ bool WebAssemblyExplicitLocals::runOnMachineFunction(MachineFunction &MF) {
         const TargetRegisterClass *RC = MRI.getRegClass(OldReg);
         Register NewReg = MRI.createVirtualRegister(RC);
         unsigned Opc = getLocalGetOpcode(RC);
-        InsertPt =
-            BuildMI(MBB, InsertPt, MI.getDebugLoc(), TII->get(Opc), NewReg)
-                .addImm(LocalId);
+        // Use a InsertPt as our DebugLoc, since MI may be discontinuous from
+        // the where this local is being inserted, causing non-linear stepping
+        // in the debugger or function entry points where variables aren't live
+        // yet. Alternative is previous instruction, but that is strictly worse
+        // since it can point at the previous statement.
+        // See crbug.com/1251909, crbug.com/1249745
+        InsertPt = BuildMI(MBB, InsertPt, InsertPt->getDebugLoc(),
+                           TII->get(Opc), NewReg).addImm(LocalId);
         MO.setReg(NewReg);
         MFI.stackifyVReg(MRI, NewReg);
         Changed = true;