[MC][DebugInfo] Emit linetable entries with known offsets immediately (#134677)

DWARF linetable entries are usually emitted as a sequence of
MCDwarfLineAddrFragment fragments containing the line-number difference
and an MCExpr describing the instruction-range the linetable entry
covers. These then get relaxed during assembly emission.

However, a large number of these instruction-range expressions are
ranges within a fixed MCDataFragment, i.e. a range over fixed-size
instructions that are not subject to relaxation at a later stage. Thus,
we can compute the address-delta immediately, and not spend time and
memory describing that computation so it can be deferred.

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
index e9295f0..7036c40 100644
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -464,6 +464,18 @@ void MCObjectStreamer::emitDwarfAdvanceLineAddr(int64_t LineDelta,
                          Label, PointerSize);
     return;
   }
+
+  // If the two labels are within the same fragment, then the address-offset is
+  // already a fixed constant and is not relaxable. Emit the advance-line-addr
+  // data immediately to save time and memory.
+  if (auto OptAddrDelta = absoluteSymbolDiff(Label, LastLabel)) {
+    SmallString<16> Tmp;
+    MCDwarfLineAddr::encode(getContext(), Assembler->getDWARFLinetableParams(),
+                            LineDelta, *OptAddrDelta, Tmp);
+    emitBytes(Tmp);
+    return;
+  }
+
   const MCExpr *AddrDelta = buildSymbolDiff(*this, Label, LastLabel, SMLoc());
   insert(getContext().allocFragment<MCDwarfLineAddrFragment>(LineDelta,
                                                              *AddrDelta));