MCFixup: Remove FKF_IsPCRel

PC-relative fixups compute their values as
`sym_a - current_location + offset` (S - P + A).
Now that targets have set PCRel at fixup creation time, we can remove
some overhead from MCAssembler::evaluateFixup.

Author: MaskRay

llvm/include/llvm/MC/MCFixup.h

@@ -75,8 +75,7 @@ class MCFixup {
 
   /// True if this is a PC-relative fixup. The relocatable expression is
   /// typically resolved When SymB is nullptr and SymA is a local symbol defined
-  /// within the current section. While MCAssembler currently sets this based on
-  /// FKF_IsPCRel, targets should ideally set it at creation.
+  /// within the current section.
   bool PCRel = false;
 
   /// Used by RISC-V style linker relaxation. Whether the fixup is

llvm/include/llvm/MC/MCFixupKindInfo.h

@@ -16,10 +16,6 @@ namespace llvm {
 /// Target independent information on a fixup kind.
 struct MCFixupKindInfo {
   enum FixupKindFlags {
-    /// Is this fixup kind PCrelative? This is used by the assembler backend to
-    /// evaluate fixup values in a target independent manner when possible.
-    FKF_IsPCRel = (1 << 0),
-
     /// Should this fixup kind force a 4-byte aligned effective PC value?
     FKF_IsAlignedDownTo32Bits = (1 << 1),
 

llvm/lib/MC/MCAsmBackend.cpp

@@ -87,22 +87,24 @@ std::optional<MCFixupKind> MCAsmBackend::getFixupKind(StringRef Name) const {
 }
 
 MCFixupKindInfo MCAsmBackend::getFixupKindInfo(MCFixupKind Kind) const {
+  // clang-format off
   static const MCFixupKindInfo Builtins[] = {
       {"FK_NONE", 0, 0, 0},
       {"FK_Data_1", 0, 8, 0},
       {"FK_Data_2", 0, 16, 0},
       {"FK_Data_4", 0, 32, 0},
       {"FK_Data_8", 0, 64, 0},
       {"FK_Data_leb128", 0, 0, 0},
-      {"FK_PCRel_1", 0, 8, MCFixupKindInfo::FKF_IsPCRel},
-      {"FK_PCRel_2", 0, 16, MCFixupKindInfo::FKF_IsPCRel},
-      {"FK_PCRel_4", 0, 32, MCFixupKindInfo::FKF_IsPCRel},
-      {"FK_PCRel_8", 0, 64, MCFixupKindInfo::FKF_IsPCRel},
+      {"FK_PCRel_1", 0, 8, 0},
+      {"FK_PCRel_2", 0, 16, 0},
+      {"FK_PCRel_4", 0, 32, 0},
+      {"FK_PCRel_8", 0, 64, 0},
       {"FK_SecRel_1", 0, 8, 0},
       {"FK_SecRel_2", 0, 16, 0},
       {"FK_SecRel_4", 0, 32, 0},
       {"FK_SecRel_8", 0, 64, 0},
   };
+  // clang-format on
 
   assert(size_t(Kind - FK_NONE) < std::size(Builtins) && "Unknown fixup kind");
   return Builtins[Kind - FK_NONE];

llvm/lib/MC/MCAssembler.cpp

@@ -161,10 +161,7 @@ bool MCAssembler::evaluateFixup(const MCFragment &F, MCFixup &Fixup,
     return true;
   }
 
-  // TODO: Require targets to set PCRel at fixup creation time.
   unsigned FixupFlags = getBackend().getFixupKindInfo(Fixup.getKind()).Flags;
-  if (FixupFlags & MCFixupKindInfo::FKF_IsPCRel)
-    Fixup.setPCRel();
   bool IsResolved = false;
   if (FixupFlags & MCFixupKindInfo::FKF_IsTarget) {
     IsResolved = getBackend().evaluateTargetFixup(Fixup, Target, Value);