CodeGen: add a missing check for bit-slice overlap in CV (#75504)

Type dereferenced fragments are specified by offset and length in bits.
The representation in CodeView is defined in terms of byte offsets. If
the bit slice overlaps at a byte that is included, we would create
invalid definition ranges.

Consider the following scenario:

~~~
01234567   01234567
---------+---------
 ====  ======
~~~

Here bits 1-4 are marked as defined as well as bits 7-9. The byte range
for the second portion overlaps and so we would say that bytes 1 and 2
are valid though there is potentially a hole. There is no way to
represent this in the defined range for the local variable in CodeView.
We simply can drop the fragment definition in such a scenario with the
variables are "optimized out".

Thanks to @rnk and @hjyamauchi for the discussion around this.

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
index 3a9d5fa..dddc08b 100644
--- a/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
@@ -1397,6 +1397,12 @@ void CodeViewDebug::calculateRanges(
     if (Location->Register == 0 || Location->LoadChain.size() > 1)
       continue;
 
+    // Codeview can only express byte-aligned offsets, ensure that we have a
+    // byte-boundaried location.
+    if (Location->FragmentInfo)
+      if (Location->FragmentInfo->OffsetInBits % 8)
+        continue;
+
     LocalVarDef DR;
     DR.CVRegister = TRI->getCodeViewRegNum(Location->Register);
     DR.InMemory = !Location->LoadChain.empty();