MC: Generate relocation for a branch crossing linker-relaxable FT_Align fragment

"Encode FT_Align in fragment's variable-size tail" or a neighbor change
caused a regression that was similar to the root cause of
ab0931b
(See the new test (.text3 with a call at the start"))

For a FT_Align fragment, the offset between location A (with offset <=
FixedSize) and B (offset == FixedSize+VarSize) cannot be resolved.

In addition, delete unneeded condition `F->isLinkerRelaxable()`.

LoongArch linker relaxation is largely under-tested, but update it as well.

Author: MaskRay

llvm/lib/MC/MCExpr.cpp

@@ -346,17 +346,16 @@ static void attemptToFoldSymbolOffsetDifference(const MCAssembler *Asm,
       Displacement *= -1;
     }
 
-    // Track whether B is before a relaxable instruction and whether A is after
-    // a relaxable instruction. If SA and SB are separated by a linker-relaxable
-    // instruction, the difference cannot be resolved as it may be changed by
-    // the linker.
+    // Track whether B is before a relaxable instruction/alignment and whether A
+    // is after a relaxable instruction/alignment. If SA and SB are separated by
+    // a linker-relaxable instruction/alignment, the difference cannot be
+    // resolved as it may be changed by the linker.
     bool BBeforeRelax = false, AAfterRelax = false;
     for (auto F = FB; F; F = F->getNext()) {
-      auto DF = F->getKind() == MCFragment::FT_Data ? F : nullptr;
-      if (DF && DF->isLinkerRelaxable()) {
-        if (&*F != FB || SBOffset != DF->getContents().size())
+      if (F && F->isLinkerRelaxable()) {
+        if (&*F != FB || SBOffset != F->getSize())
           BBeforeRelax = true;
-        if (&*F != FA || SAOffset == DF->getContents().size())
+        if (&*F != FA || SAOffset == F->getSize())
           AAfterRelax = true;
         if (BBeforeRelax && AAfterRelax)
           return;
@@ -370,17 +369,15 @@ static void attemptToFoldSymbolOffsetDifference(const MCAssembler *Asm,
       }
 
       int64_t Num;
-      if (DF) {
-        Displacement += DF->getContents().size();
-      } else if (F->getKind() == MCFragment::FT_Relaxable &&
+      if (F->getKind() == MCFragment::FT_Data) {
+        Displacement += F->getFixedSize();
+      } else if ((F->getKind() == MCFragment::FT_Relaxable ||
+                  F->getKind() == MCFragment::FT_Align) &&
                  Asm->hasFinalLayout()) {
         // Before finishLayout, a relaxable fragment's size is indeterminate.
         // After layout, during relocation generation, it can be treated as a
         // data fragment.
         Displacement += F->getSize();
-      } else if (F->getKind() == MCFragment::FT_Align && Layout &&
-                 F->isLinkerRelaxable()) {
-        Displacement += Asm->computeFragmentSize(*F);
       } else if (auto *FF = dyn_cast<MCFillFragment>(F);
                  FF && FF->getNumValues().evaluateAsAbsolute(Num)) {
         Displacement += Num * FF->getValueSize();

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

@@ -254,6 +254,7 @@ bool LoongArchAsmBackend::relaxAlign(MCFragment &F, unsigned &Size) {
   MCFixup Fixup =
       MCFixup::create(0, Expr, FirstLiteralRelocationKind + ELF::R_LARCH_ALIGN);
   F.setVarFixups({Fixup});
+  F.setLinkerRelaxable();
   F.getParent()->setLinkerRelaxable();
   return true;
 }

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

@@ -320,6 +320,7 @@ bool RISCVAsmBackend::relaxAlign(MCFragment &F, unsigned &Size) {
   MCFixup Fixup =
       MCFixup::create(0, Expr, FirstLiteralRelocationKind + ELF::R_RISCV_ALIGN);
   F.setVarFixups({Fixup});
+  F.setLinkerRelaxable();
   F.getParent()->setLinkerRelaxable();
   return true;
 }

llvm/test/MC/RISCV/Relocations/mc-dump.s

@@ -9,10 +9,10 @@
 # CHECK-NEXT:0 Data LinkerRelaxable Size:8 [97,00,00,00,e7,80,00,00]
 # CHECK-NEXT:  Fixup @0 Value:specifier(19,ext) Kind:4023
 # CHECK-NEXT:  Symbol @0 $x
-# CHECK-NEXT:8 Align Size:0+4 []
+# CHECK-NEXT:8 Align LinkerRelaxable Size:0+4 []
 # CHECK-NEXT:  Align:8 Fill:0 FillLen:1 MaxBytesToEmit:8 Nops
 # CHECK-NEXT:  Fixup @0 Value:4 Kind:[[#]]
-# CHECK-NEXT:12 Align Size:4+4 [13,05,30,00]
+# CHECK-NEXT:12 Align LinkerRelaxable Size:4+4 [13,05,30,00]
 # CHECK-NEXT:  Align:8 Fill:0 FillLen:1 MaxBytesToEmit:8 Nops
 # CHECK-NEXT:  Fixup @4 Value:4 Kind:[[#]]
 # CHECK-NEXT:]

llvm/test/MC/RISCV/align.s

@@ -154,3 +154,41 @@ data2:
 2:
 	bnez t1, 1b
 	bnez t1, 2b
+
+## .text3 with a call at the start
+# NORELAX-RELOC:    .rela.text3a
+# C-OR-ZCA-EXT-NORELAX-RELOC: .rela.text3a
+# RELAX-RELOC:      .rela.text3a {
+# RELAX-RELOC-NEXT:    0x0  R_RISCV_CALL_PLT foo 0x0
+# RELAX-RELOC-NEXT:    0x0  R_RISCV_RELAX - 0x0
+# RELAX-RELOC-NEXT:    0xC  R_RISCV_BRANCH .Ltmp[[#]] 0x0
+# RELAX-RELOC-NEXT:    0x10 R_RISCV_ALIGN - 0x4
+# RELAX-RELOC-NEXT:    0x14 R_RISCV_BRANCH .Ltmp[[#]] 0x0
+# RELAX-RELOC-NEXT: }
+.section .text3a, "ax"
+call foo
+bnez t1, 1f
+bnez t2, 2f
+1:
+.p2align 3
+2:
+bnez t1, 1b
+bnez t1, 2b
+
+## .text3 with a call at the end
+# RELAX-RELOC:      .rela.text3b {
+# 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:    0x14 R_RISCV_CALL_PLT foo 0x0
+# RELAX-RELOC-NEXT:    0x14 R_RISCV_RELAX - 0x0
+# RELAX-RELOC-NEXT: }
+.section .text3b, "ax"
+bnez t1, 1f
+bnez t2, 2f
+1:
+.p2align 3
+2:
+bnez t1, 1b
+bnez t1, 2b
+call foo