[hexagon] Prevent alignment search beyond a label (llvm#130631)

When searching for packets to .align, don't consider ones which would
require padding beyond a label.

There are two problems with padding beyond a label:
- the distance between labels may increase for some offsets to become
too large;
- u/sleb128 values that encode a difference will not be updated because
they are computed before the align command is handled.

This is more a short-term fix/hack. The proper solution would be to
unify `.align` and `.falign` handling and move it to the layout loop.

(cherry picked from commit 1fe4631)

Author: quic-akaryaki

llvm/lib/Target/Hexagon/MCTargetDesc/HexagonAsmBackend.cpp

@@ -728,6 +728,24 @@ class HexagonAsmBackend : public MCAsmBackend {
               MCContext &Context = Asm.getContext();
               auto &RF = cast<MCRelaxableFragment>(*Frags[K]);
               auto &Inst = const_cast<MCInst &>(RF.getInst());
+
+              const bool WouldTraverseLabel = llvm::any_of(
+                  Asm.symbols(), [&Asm, &RF, &Inst](MCSymbol const &sym) {
+                    uint64_t Offset = 0;
+                    const bool HasOffset = Asm.getSymbolOffset(sym, Offset);
+                    const unsigned PacketSizeBytes =
+                        HexagonMCInstrInfo::bundleSize(Inst) *
+                        HEXAGON_INSTR_SIZE;
+                    const bool OffsetPastSym =
+                        Offset <= (Asm.getFragmentOffset(RF) + PacketSizeBytes);
+                    return !sym.isVariable() && Offset != 0 && HasOffset &&
+                           OffsetPastSym;
+                  });
+              if (WouldTraverseLabel) {
+                Size = 0;
+                break;
+              }
+
               while (Size > 0 &&
                      HexagonMCInstrInfo::bundleSize(Inst) < MaxPacketSize) {
                 MCInst *Nop = Context.createMCInst();

llvm/test/MC/Hexagon/align-leb128.s

@@ -0,0 +1,18 @@
+# RUN: llvm-mc -triple=hexagon -filetype=obj %s | llvm-readelf -x .data - \
+# RUN:   | FileCheck %s --match-full-lines
+
+# Illustrate the case when padding packets across labels also breaks leb128
+# relocations. This happens because .align padding is inserted once at the
+# very end of the section layout.
+L1:
+  nop
+L2:
+.size L1, L2-L1
+.align 16
+  nop
+.data
+.word L2-L1
+.uleb128 L2-L1
+
+# CHECK: Hex dump of section '.data':
+# CHECK-NEXT: 0x00000000 04000000 04 .....

llvm/test/MC/Hexagon/align.s

@@ -58,3 +58,16 @@ r0 = vextract(v0, r0)
   r1 = sub (##1, r1) }
 .align 16
 { r0 = sub (#1, r0) }
+
+# Don't search backwards to pad packets beyond a label:
+{ r1 = add(r1, r0) }
+# CHECK-NEXT: { r1 = add(r1,r0)
+# CHECK-NOT:  nop
+
+post_label:
+.align 16
+# CHECK-LABEL: post_label
+# CHECK-NEXT: { nop
+# CHECK-NEXT:   nop }
+# CHECK-NEXT: { r1 = sub(#1,r1) }
+{ r1 = sub(#1, r1) }