[llvm] [LoongArch] Insert nops and emit align reloc when handle alignment directive (PR #72962)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-backend-loongarch

@llvm/pr-subscribers-mc

Author: Jinyang He (MQ-mengqing)

<details>
<summary>Changes</summary>

Refer to RISCV, we will fix up the alignment if linker relaxation changes code size and breaks alignment. Insert enough Nops and emit R_LARCH_ALIGN relocation type so that linker could satisfy the alignment by removing Nops. It does so only in sections with the SHF_EXECINSTR flag.

---
Full diff: https://github.com/llvm/llvm-project/pull/72962.diff


4 Files Affected:

- (modified) llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.cpp (+48) 
- (modified) llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.h (+9) 
- (modified) llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchFixupKinds.h (+3-1) 
- (added) llvm/test/MC/LoongArch/Relocations/relax-align.s (+28) 


``````````diff
diff --git a/llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.cpp b/llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.cpp
index a35916d2ad2197d..6625ad6bc94d188 100644
--- a/llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.cpp
+++ b/llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.cpp
@@ -160,6 +160,54 @@ void LoongArchAsmBackend::applyFixup(const MCAssembler &Asm,
   }
 }
 
+// Linker relaxation may change code size. We have to insert Nops
+// for .align directive when linker relaxation enabled. So then Linker
+// could satisfy alignment by removing Nops.
+// The function returns the total Nops Size we need to insert.
+bool LoongArchAsmBackend::shouldInsertExtraNopBytesForCodeAlign(
+    const MCAlignFragment &AF, unsigned &Size) {
+  // Calculate Nops Size only when linker relaxation enabled.
+  const MCSubtargetInfo *STI = AF.getSubtargetInfo();
+  if (!STI->hasFeature(LoongArch::FeatureRelax))
+    return false;
+
+  const unsigned MinNopLen = 4;
+  Size = AF.getAlignment().value() - MinNopLen;
+  return AF.getAlignment() > MinNopLen;
+}
+
+// We need to insert R_LARCH_ALIGN relocation type to indicate the
+// position of Nops and the total bytes of the Nops have been inserted
+// when linker relaxation enabled.
+// The function inserts fixup_loongarch_align fixup which eventually will
+// transfer to R_LARCH_ALIGN relocation type.
+bool LoongArchAsmBackend::shouldInsertFixupForCodeAlign(
+    MCAssembler &Asm, const MCAsmLayout &Layout, MCAlignFragment &AF) {
+  // Insert the fixup only when linker relaxation enabled.
+  const MCSubtargetInfo *STI = AF.getSubtargetInfo();
+  if (!STI->hasFeature(LoongArch::FeatureRelax))
+    return false;
+
+  // Calculate total Nops we need to insert. If there are none to insert
+  // then simply return.
+  unsigned Count;
+  if (!shouldInsertExtraNopBytesForCodeAlign(AF, Count))
+    return false;
+
+  const MCExpr *Dummy = MCConstantExpr::create(0, Asm.getContext());
+  // Create fixup_loongarch_align fixup.
+  MCFixup Fixup = MCFixup::create(
+      0, Dummy, MCFixupKind(LoongArch::fixup_loongarch_align), SMLoc());
+
+  uint64_t FixedValue = 0;
+  MCValue NopBytes = MCValue::get(Count);
+
+  Asm.getWriter().recordRelocation(Asm, Layout, &AF, Fixup, NopBytes,
+                                   FixedValue);
+
+  return true;
+}
+
 bool LoongArchAsmBackend::shouldForceRelocation(const MCAssembler &Asm,
                                                 const MCFixup &Fixup,
                                                 const MCValue &Target) {
diff --git a/llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.h b/llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.h
index f840f9fa2b6a007..1f4743f34dad2a7 100644
--- a/llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.h
+++ b/llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.h
@@ -40,6 +40,15 @@ class LoongArchAsmBackend : public MCAsmBackend {
                   uint64_t Value, bool IsResolved,
                   const MCSubtargetInfo *STI) const override;
 
+  // Return Size with extra Nop Bytes for alignment directive in code section.
+  bool shouldInsertExtraNopBytesForCodeAlign(const MCAlignFragment &AF,
+                                             unsigned &Size) override;
+
+  // Insert target specific fixup type for alignment directive in code section.
+  bool shouldInsertFixupForCodeAlign(MCAssembler &Asm,
+                                     const MCAsmLayout &Layout,
+                                     MCAlignFragment &AF) override;
+
   bool shouldForceRelocation(const MCAssembler &Asm, const MCFixup &Fixup,
                              const MCValue &Target) override;
 
diff --git a/llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchFixupKinds.h b/llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchFixupKinds.h
index ba2d6718cdf9a27..8d773b649107ed6 100644
--- a/llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchFixupKinds.h
+++ b/llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchFixupKinds.h
@@ -106,7 +106,9 @@ enum Fixups {
   // 20-bit fixup corresponding to %gd_pc_hi20(foo) for instruction pcalau12i.
   fixup_loongarch_tls_gd_pc_hi20,
   // 20-bit fixup corresponding to %gd_hi20(foo) for instruction lu12i.w.
-  fixup_loongarch_tls_gd_hi20
+  fixup_loongarch_tls_gd_hi20,
+  // Generate an R_LARCH_ALIGN which indicates the linker may fixup align here.
+  fixup_loongarch_align = FirstLiteralRelocationKind + ELF::R_LARCH_ALIGN
 };
 } // end namespace LoongArch
 } // end namespace llvm
diff --git a/llvm/test/MC/LoongArch/Relocations/relax-align.s b/llvm/test/MC/LoongArch/Relocations/relax-align.s
new file mode 100644
index 000000000000000..36524182777cb02
--- /dev/null
+++ b/llvm/test/MC/LoongArch/Relocations/relax-align.s
@@ -0,0 +1,28 @@
+# RUN: llvm-mc --filetype=obj --triple=loongarch64 %s -o %t
+# RUN: llvm-readobj -r %t | FileCheck %s
+# RUN: llvm-mc --filetype=obj --triple=loongarch64 -mattr=+relax %s -o %t
+# RUN: llvm-readobj -r %t | FileCheck %s --check-prefix=CHECKR
+
+# CHECKR:       Relocations [
+# CHECKR-NEXT:    Section ({{.*}}) .rela.text {
+# CHECKR-NEXT:      0x0 R_LARCH_ALIGN - 0xC
+# CHECKR-NEXT:      0x10 R_LARCH_ALIGN - 0x1C
+# CHECKR-NEXT:      0x2C R_LARCH_ALIGN - 0xC
+# CHECKR-NEXT:    }
+# CHECKR-NEXT:  ]
+
+# CHECK:       Relocations [
+# CHECK-NEXT:  ]
+
+.text
+.p2align 4
+nop
+.p2align 5
+.p2align 4
+nop
+## Not emit R_LARCH_ALIGN if code alignment great than alignment directive.
+.p2align 2
+.p2align 1
+.p2align 0
+## Not emit R_LARCH_ALIGN if alignment directive with specific padding value.
+.p2align 4, 1

``````````

</details>


https://github.com/llvm/llvm-project/pull/72962