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

[Alexey Karyakin via llvm-commits]

https://github.com/quic-akaryaki created https://github.com/llvm/llvm-project/pull/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.

>From 608bae55f11e7c6a20dc0647ee39f59e055d2368 Mon Sep 17 00:00:00 2001
From: Alexey Karyakin <akaryaki at quicinc.com>
Date: Mon, 10 Mar 2025 09:14:11 -0700
Subject: [PATCH] [hexagon] Prevent alignment search beyond a label

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.
---
 .../Hexagon/MCTargetDesc/HexagonAsmBackend.cpp | 18 ++++++++++++++++++
 llvm/test/MC/Hexagon/align-leb128.s            | 18 ++++++++++++++++++
 llvm/test/MC/Hexagon/align.s                   | 13 +++++++++++++
 3 files changed, 49 insertions(+)
 create mode 100644 llvm/test/MC/Hexagon/align-leb128.s

diff --git a/llvm/lib/Target/Hexagon/MCTargetDesc/HexagonAsmBackend.cpp b/llvm/lib/Target/Hexagon/MCTargetDesc/HexagonAsmBackend.cpp
index 98b1dde8fa3fc..725067e0c9bdd 100644
--- a/llvm/lib/Target/Hexagon/MCTargetDesc/HexagonAsmBackend.cpp
+++ b/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();
diff --git a/llvm/test/MC/Hexagon/align-leb128.s b/llvm/test/MC/Hexagon/align-leb128.s
new file mode 100644
index 0000000000000..65cbf6c24cf07
--- /dev/null
+++ b/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
diff --git a/llvm/test/MC/Hexagon/align.s b/llvm/test/MC/Hexagon/align.s
index 9c2978df71373..e17d09cfd8c96 100644
--- a/llvm/test/MC/Hexagon/align.s
+++ b/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) }