[DebugInfo] Correctly handle salvaged casts and split fragments at ISel

Previously, LLVM had no functional way of performing casts inside of a
DIExpression(), which made salvaging cast instructions other than Noop
casts impossible. This patch enables the salvaging of casts by using the
DW_OP_LLVM_convert operator for SExt and Trunc instructions.

There is another issue which is exposed by this fix, in which fragment
DIExpressions (which are preserved more readily by this patch) for
values that must be split across registers in ISel trigger an assertion,
as the 'split' fragments extend beyond the bounds of the fragment
DIExpression causing an error. This patch also fixes this issue by
checking the fragment status of DIExpressions which are to be split, and
dropping fragments that are invalid.

1 files changed, 17 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index d5690a0..b2d511c 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -1676,9 +1676,8 @@ DIExpression *llvm::salvageDebugInfoImpl(Instruction &I,
   };
 
   // initializer-list helper for applying operators to the source DIExpression.
-  auto applyOps =
-      [&](std::initializer_list<uint64_t> Opcodes) -> DIExpression * {
-    SmallVector<uint64_t, 8> Ops(Opcodes);
+  auto applyOps = [&](ArrayRef<uint64_t> Opcodes) -> DIExpression * {
+    SmallVector<uint64_t, 8> Ops(Opcodes.begin(), Opcodes.end());
     return doSalvage(Ops);
   };
 
@@ -1686,8 +1685,21 @@ DIExpression *llvm::salvageDebugInfoImpl(Instruction &I,
     // No-op casts and zexts are irrelevant for debug info.
     if (CI->isNoopCast(DL) || isa<ZExtInst>(&I))
       return SrcDIExpr;
-    return nullptr;
-  } else if (auto *GEP = dyn_cast<GetElementPtrInst>(&I)) {
+
+    Type *Type = CI->getType();
+    // Casts other than Trunc or SExt to scalar types cannot be salvaged.
+    if (Type->isVectorTy() || (!isa<TruncInst>(&I) && !isa<SExtInst>(&I)))
+      return nullptr;
+
+    Value *FromValue = CI->getOperand(0);
+    unsigned FromTypeBitSize = FromValue->getType()->getScalarSizeInBits();
+    unsigned ToTypeBitSize = Type->getScalarSizeInBits();
+
+    return applyOps(DIExpression::getExtOps(FromTypeBitSize, ToTypeBitSize,
+                                            isa<SExtInst>(&I)));
+  }
+
+  if (auto *GEP = dyn_cast<GetElementPtrInst>(&I)) {
     unsigned BitWidth =
         M.getDataLayout().getIndexSizeInBits(GEP->getPointerAddressSpace());
     // Rewrite a constant GEP into a DIExpression.