[LoongArch] Use R_LARCH_ALIGN without symbol as much as possible (#93775)

To support the third parameter of the alignment directive, R_LARCH_ALIGN
relocations need a non-zero symbol index.
In many cases we don't need the third parameter and can set the symbol
index to 0.
This patch will remove a lot of .Lla-relax-align* symbols and mitigate
the size regression due to
#72962.

Co-authored-by: Jinyang He <[email protected]>
Co-authored-by: Weining Lu <[email protected]>

Author: SixWeining

lld/test/ELF/loongarch-relax-emit-relocs.s

@@ -25,7 +25,7 @@
 # CHECK-NEXT:     R_LARCH_PCALA_LO12 _start
 # CHECK-NEXT:     R_LARCH_RELAX *ABS*
 # CHECK-NEXT:   nop
-# CHECK-NEXT:     R_LARCH_ALIGN .Lla-relax-align0+0x4
+# CHECK-NEXT:     R_LARCH_ALIGN *ABS*+0xc
 # CHECK-NEXT:   nop
 # CHECK-NEXT:   ret
 
@@ -37,7 +37,7 @@
 # CHECKR-NEXT:     R_LARCH_PCALA_LO12 _start
 # CHECKR-NEXT:     R_LARCH_RELAX *ABS*
 # CHECKR-NEXT:   nop
-# CHECKR-NEXT:     R_LARCH_ALIGN .Lla-relax-align0+0x4
+# CHECKR-NEXT:     R_LARCH_ALIGN *ABS*+0xc
 # CHECKR-NEXT:   nop
 # CHECKR-NEXT:   nop
 # CHECKR-NEXT:   ret

llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.cpp

@@ -214,8 +214,8 @@ bool LoongArchAsmBackend::shouldInsertFixupForCodeAlign(
 
   // Calculate total Nops we need to insert. If there are none to insert
   // then simply return.
-  unsigned Count;
-  if (!shouldInsertExtraNopBytesForCodeAlign(AF, Count))
+  unsigned InsertedNopBytes;
+  if (!shouldInsertExtraNopBytesForCodeAlign(AF, InsertedNopBytes))
     return false;
 
   MCSection *Sec = AF.getParent();
@@ -224,20 +224,26 @@ bool LoongArchAsmBackend::shouldInsertFixupForCodeAlign(
   // Create fixup_loongarch_align fixup.
   MCFixup Fixup =
       MCFixup::create(0, Dummy, MCFixupKind(LoongArch::fixup_loongarch_align));
-  const MCSymbolRefExpr *MCSym = getSecToAlignSym()[Sec];
-  if (MCSym == nullptr) {
-    // Create a symbol and make the value of symbol is zero.
-    MCSymbol *Sym = Ctx.createNamedTempSymbol("la-relax-align");
-    Sym->setFragment(&*Sec->getBeginSymbol()->getFragment());
-    Asm.registerSymbol(*Sym);
-    MCSym = MCSymbolRefExpr::create(Sym, Ctx);
-    getSecToAlignSym()[Sec] = MCSym;
-  }
+  unsigned MaxBytesToEmit = AF.getMaxBytesToEmit();
+
+  auto createExtendedValue = [&]() {
+    const MCSymbolRefExpr *MCSym = getSecToAlignSym()[Sec];
+    if (MCSym == nullptr) {
+      // Define a marker symbol at the section with an offset of 0.
+      MCSymbol *Sym = Ctx.createNamedTempSymbol("la-relax-align");
+      Sym->setFragment(&*Sec->getBeginSymbol()->getFragment());
+      Asm.registerSymbol(*Sym);
+      MCSym = MCSymbolRefExpr::create(Sym, Ctx);
+      getSecToAlignSym()[Sec] = MCSym;
+    }
+    return MCValue::get(MCSym, nullptr,
+                        MaxBytesToEmit << 8 | Log2(AF.getAlignment()));
+  };
 
   uint64_t FixedValue = 0;
-  unsigned Lo = Log2_64(Count) + 1;
-  unsigned Hi = AF.getMaxBytesToEmit() >= Count ? 0 : AF.getMaxBytesToEmit();
-  MCValue Value = MCValue::get(MCSym, nullptr, Hi << 8 | Lo);
+  MCValue Value = MaxBytesToEmit >= InsertedNopBytes
+                      ? MCValue::get(InsertedNopBytes)
+                      : createExtendedValue();
   Asm.getWriter().recordRelocation(Asm, Layout, &AF, Fixup, Value, FixedValue);
 
   return true;

llvm/test/MC/LoongArch/Relocations/relax-addsub.s

@@ -28,7 +28,7 @@
 
 # RELAX:       Relocations [
 # RELAX-NEXT:    Section ({{.*}}) .rela.text {
-# RELAX-NEXT:      0x4 R_LARCH_ALIGN {{.*}} 0x4
+# RELAX-NEXT:      0x4 R_LARCH_ALIGN - 0xC
 # RELAX-NEXT:      0x10 R_LARCH_PCALA_HI20 .L1 0x0
 # RELAX-NEXT:      0x10 R_LARCH_RELAX - 0x0
 # RELAX-NEXT:      0x14 R_LARCH_PCALA_LO12 .L1 0x0

llvm/test/MC/LoongArch/Relocations/relax-align.s

@@ -63,17 +63,19 @@ ret
 ## Test the symbol index is different from .text.
 .section .text2, "ax"
 .p2align 4
+.p2align 4, , 4
 break 7
 
 # RELOC:            Relocations [
 # RELAX-RELOC-NEXT:   Section ({{.*}}) .rela.text {
-# RELAX-RELOC-NEXT:     0x24 R_LARCH_ALIGN .Lla-relax-align0 0x4
-# RELAX-RELOC-NEXT:     0x34 R_LARCH_ALIGN .Lla-relax-align0 0x5
-# RELAX-RELOC-NEXT:     0x50 R_LARCH_ALIGN .Lla-relax-align0 0x4
+# RELAX-RELOC-NEXT:     0x24 R_LARCH_ALIGN - 0xC
+# RELAX-RELOC-NEXT:     0x34 R_LARCH_ALIGN - 0x1C
+# RELAX-RELOC-NEXT:     0x50 R_LARCH_ALIGN - 0xC
 # RELAX-RELOC-NEXT:     0x60 R_LARCH_ALIGN .Lla-relax-align0 0xB04
-# RELAX-RELOC-NEXT:     0x70 R_LARCH_ALIGN .Lla-relax-align0 0x4
+# RELAX-RELOC-NEXT:     0x70 R_LARCH_ALIGN - 0xC
 # RELAX-RELOC-NEXT:   }
 # RELAX-RELOC-NEXT:   Section ({{.*}}) .rela.text2 {
-# RELAX-RELOC-NEXT:     0x0 R_LARCH_ALIGN .Lla-relax-align1 0x4
+# RELAX-RELOC-NEXT:     0x0 R_LARCH_ALIGN - 0xC
+# RELAX-RELOC-NEXT:     0xC R_LARCH_ALIGN .Lla-relax-align1 0x404
 # RELAX-RELOC-NEXT:   }
 # RELOC-NEXT:       ]