[MC] Restore a special case to support limited A-B folding when A/B are in the same fragment being laided out

Add subsection-if.s to test what we can fold (in the same fragment) and what we cannot.

Fix https://github.com/ClangBuiltLinux/linux/issues/1876
Fixes: 4bdc7f7a331f82cca1637388cf68bdc5b32ab43b

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/MC/MCExpr.cpp b/llvm/lib/MC/MCExpr.cpp
index 46a0367..df305c8 100644
--- a/llvm/lib/MC/MCExpr.cpp
+++ b/llvm/lib/MC/MCExpr.cpp
@@ -625,6 +625,12 @@ static void AttemptToFoldSymbolOffsetDifference(
     return;
 
   if (Layout) {
+    // If both symbols are in the same fragment, return the difference of their
+    // offsets. canGetFragmentOffset(FA) may be false.
+    if (FA == FB && !SA.isVariable() && !SB.isVariable()) {
+      Addend += SA.getOffset() - SB.getOffset();
+      return FinalizeFolding();
+    }
     // One of the symbol involved is part of a fragment being laid out. Quit now
     // to avoid a self loop.
     if (!Layout->canGetFragmentOffset(FA) || !Layout->canGetFragmentOffset(FB))