MC: Set MCSection::LinkerRelaxable for linker-relaxable MCAlignFragment

Commit bb03cdc caused a Linux kernel
regression ClangBuiltLinux/linux#2091

When a section contains linker-relaxable MCAlignmentFragment but no
linker-relaxable instructions, the RISCVAsmBackend::isPCRelFixupResolved
code path should be taken as well. The #76552 condition in the fragment
walk code will make the fixup unresolvable, leading to a relocation.

Author: MaskRay

llvm/lib/MC/MCObjectStreamer.cpp

@@ -586,7 +586,14 @@ void MCObjectStreamer::emitCodeAlignment(Align Alignment,
                                          const MCSubtargetInfo *STI,
                                          unsigned MaxBytesToEmit) {
   emitValueToAlignment(Alignment, 0, 1, MaxBytesToEmit);
-  cast<MCAlignFragment>(getCurrentFragment())->setEmitNops(true, STI);
+  auto *F = cast<MCAlignFragment>(getCurrentFragment());
+  F->setEmitNops(true, STI);
+  // With RISC-V style linker relaxation, mark the section as linker-relaxable
+  // if the alignment is larger than the minimum NOP size.
+  unsigned Size;
+  if (getAssembler().getBackend().shouldInsertExtraNopBytesForCodeAlign(*F,
+                                                                        Size))
+    getCurrentSectionOnly()->setLinkerRelaxable();
 }
 
 void MCObjectStreamer::emitValueToOffset(const MCExpr *Offset,

llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp

@@ -546,7 +546,7 @@ static uint64_t adjustFixupValue(const MCFixup &Fixup, uint64_t Value,
 
 bool RISCVAsmBackend::isPCRelFixupResolved(const MCSymbol *SymA,
                                            const MCFragment &F) {
-  // If the section does not contain linker-relaxable instructions, PC-relative
+  // If the section does not contain linker-relaxable fragments, PC-relative
   // fixups can be resolved.
   if (!F.getParent()->isLinkerRelaxable())
     return true;

llvm/test/MC/RISCV/align.s

@@ -134,3 +134,23 @@ data2:
 	.option norvc
 	.balign 4
 	add	a0, a0, a1
+
+## Branches crossing the linker-relaxable R_RISCV_ALIGN need relocations.
+# RELAX-RELOC:      .rela.text3 {
+# RELAX-RELOC-NEXT:    0x4 R_RISCV_BRANCH .Ltmp[[#]] 0x0
+# RELAX-RELOC-NEXT:    0x8 R_RISCV_ALIGN - 0x4
+# RELAX-RELOC-NEXT:    0xC R_RISCV_BRANCH .Ltmp[[#]] 0x0
+# RELAX-RELOC-NEXT: }
+# C-OR-ZCA-EXT-RELAX-RELOC:      .rela.text3 {
+# C-OR-ZCA-EXT-RELAX-RELOC-NEXT:    0x4 R_RISCV_BRANCH .Ltmp[[#]] 0x0
+# C-OR-ZCA-EXT-RELAX-RELOC-NEXT:    0x8 R_RISCV_ALIGN - 0x4
+# C-OR-ZCA-EXT-RELAX-RELOC-NEXT:    0xC R_RISCV_BRANCH .Ltmp[[#]] 0x0
+# C-OR-ZCA-EXT-RELAX-RELOC-NEXT: }
+	.section .text3, "ax"
+	bnez t1, 1f
+	bnez t2, 2f
+1:
+	.p2align 3
+2:
+	bnez t1, 1b
+	bnez t1, 2b

llvm/test/MC/RISCV/cfi-advance.s

@@ -13,16 +13,26 @@
 # RELAX:        R_RISCV_CALL_PLT
 # RELAX-NEXT:   R_RISCV_RELAX
 # RELAX-EMPTY:
-# CHECK-NEXT:   Relocation section '.rela.eh_frame' at offset {{.*}} contains 1 entries:
-# CHECK:         Offset     Info    Type                Sym. Value  Symbol's Name + Addend
-# CHECK-NEXT:   0000001c  00000139 R_RISCV_32_PCREL       00000000   .L0 + 0
+# NORELAX-NEXT: Relocation section '.rela.eh_frame' at offset {{.*}} contains 1 entries:
+# NORELAX:       Offset     Info    Type                Sym. Value  Symbol's Name + Addend
+# NORELAX-NEXT: 0000001c  00000139 R_RISCV_32_PCREL       00000000   .L0 + 0
+# RELAX-NEXT:   Relocation section '.rela.eh_frame' at offset {{.*}} contains 5 entries:
+# RELAX:         Offset     Info    Type                Sym. Value  Symbol's Name + Addend
+# RELAX-NEXT:   0000001c  00000139 R_RISCV_32_PCREL       00000000   .L0  + 0
+# RELAX-NEXT:   00000020  00000c23 R_RISCV_ADD32          0001017a   .L0  + 0
+# RELAX-NEXT:   00000020  00000127 R_RISCV_SUB32          00000000   .L0  + 0
+# RELAX-NEXT:   00000035  00000b35 R_RISCV_SET6           00010176   .L0  + 0
+# RELAX-NEXT:   00000035  00000934 R_RISCV_SUB6           0001016e   .L0  + 0
 # CHECK-EMPTY:
-# CHECK:        Symbol table '.symtab' contains 13 entries:
-# CHECK-NEXT:      Num:    Value  Size Type    Bind   Vis       Ndx Name
-# CHECK-NEXT:        0: 00000000     0 NOTYPE  LOCAL  DEFAULT   UND
-# CHECK-NEXT:        1: 00000000     0 NOTYPE  LOCAL  DEFAULT     2 .L0 {{$}}
-# CHECK:             3: 00000004     0 NOTYPE  LOCAL  DEFAULT     2 .L0{{$}}
-# CHECK-NOT:    .L0
+# NORELAX:      Symbol table '.symtab' contains 13 entries:
+# RELAX:        Symbol table '.symtab' contains 16 entries:
+# RELAX-NEXT:      Num:    Value  Size Type    Bind   Vis       Ndx Name
+# RELAX-NEXT:        0: 00000000     0 NOTYPE  LOCAL  DEFAULT   UND
+# RELAX-NEXT:        1: 00000000     0 NOTYPE  LOCAL  DEFAULT     2 .L0 {{$}}
+# RELAX:             3: 00000004     0 NOTYPE  LOCAL  DEFAULT     2 .L0{{$}}
+# RELAX:             9: 0001016e     0 NOTYPE  LOCAL  DEFAULT     2 .L0 {{$}}
+# RELAX:            11: 00010176     0 NOTYPE  LOCAL  DEFAULT     2 .L0 {{$}}
+# RELAX:            12: 0001017a     0 NOTYPE  LOCAL  DEFAULT     2 .L0 {{$}}
 
 # CHECK-DWARFDUMP: DW_CFA_advance_loc1: 104
 # CHECK-DWARFDUMP-NEXT: DW_CFA_def_cfa_offset: +8