[MC][ARM] Resolve some pcrel fixups at assembly time (PR44929)

MC currently does not emit these relocation types, and lld does not
handle them. Add FKF_Constant as a work-around of some ARM code after
D72197. Eventually we probably should implement these relocation types.

By Fangrui Song!

Differential revision: https://reviews.llvm.org/D72892

include/llvm/MC/MCFixupKindInfo.h

@@ -22,7 +22,12 @@ struct MCFixupKindInfo {
     FKF_IsAlignedDownTo32Bits = (1 << 1),
 
     /// Should this fixup be evaluated in a target dependent manner?
-    FKF_IsTarget = (1 << 2)
+    FKF_IsTarget = (1 << 2),
+
+    /// This fixup kind should be resolved if defined.
+    /// FIXME This is a workaround because we don't support certain ARM
+    /// relocation types. This flag should eventually be removed.
+    FKF_Constant = 1 << 3,
   };
 
   /// A target specific name for the fixup kind. The names will be unique for

lib/MC/MCAssembler.cpp

@@ -224,6 +224,7 @@ bool MCAssembler::evaluateFixup(const MCAsmLayout &Layout,
     return getBackend().evaluateTargetFixup(*this, Layout, Fixup, DF, Target,
                                             Value, WasForced);
 
+  unsigned FixupFlags = getBackendPtr()->getFixupKindInfo(Fixup.getKind()).Flags;
   bool IsPCRel = getBackendPtr()->getFixupKindInfo(Fixup.getKind()).Flags &
                  MCFixupKindInfo::FKF_IsPCRel;
 
@@ -239,8 +240,9 @@ bool MCAssembler::evaluateFixup(const MCAsmLayout &Layout,
       if (A->getKind() != MCSymbolRefExpr::VK_None || SA.isUndefined()) {
         IsResolved = false;
       } else if (auto *Writer = getWriterPtr()) {
-        IsResolved = Writer->isSymbolRefDifferenceFullyResolvedImpl(
-            *this, SA, *DF, false, true);
+        IsResolved = (FixupFlags & MCFixupKindInfo::FKF_Constant) ||
+                     Writer->isSymbolRefDifferenceFullyResolvedImpl(
+                         *this, SA, *DF, false, true);
       }
     }
   } else {

lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp

@@ -55,31 +55,29 @@ Optional<MCFixupKind> ARMAsmBackend::getFixupKind(StringRef Name) const {
 }
 
 const MCFixupKindInfo &ARMAsmBackend::getFixupKindInfo(MCFixupKind Kind) const {
+  unsigned IsPCRelConstant =
+      MCFixupKindInfo::FKF_IsPCRel | MCFixupKindInfo::FKF_Constant;
   const static MCFixupKindInfo InfosLE[ARM::NumTargetFixupKinds] = {
       // This table *must* be in the order that the fixup_* kinds are defined in
       // ARMFixupKinds.h.
       //
       // Name                      Offset (bits) Size (bits)     Flags
-      {"fixup_arm_ldst_pcrel_12", 0, 32, MCFixupKindInfo::FKF_IsPCRel},
+      {"fixup_arm_ldst_pcrel_12", 0, 32, IsPCRelConstant},
       {"fixup_t2_ldst_pcrel_12", 0, 32,
-       MCFixupKindInfo::FKF_IsPCRel |
-           MCFixupKindInfo::FKF_IsAlignedDownTo32Bits},
-      {"fixup_arm_pcrel_10_unscaled", 0, 32, MCFixupKindInfo::FKF_IsPCRel},
-      {"fixup_arm_pcrel_10", 0, 32, MCFixupKindInfo::FKF_IsPCRel},
+       IsPCRelConstant | MCFixupKindInfo::FKF_IsAlignedDownTo32Bits},
+      {"fixup_arm_pcrel_10_unscaled", 0, 32, IsPCRelConstant},
+      {"fixup_arm_pcrel_10", 0, 32, IsPCRelConstant},
       {"fixup_t2_pcrel_10", 0, 32,
        MCFixupKindInfo::FKF_IsPCRel |
            MCFixupKindInfo::FKF_IsAlignedDownTo32Bits},
       {"fixup_arm_pcrel_9", 0, 32, MCFixupKindInfo::FKF_IsPCRel},
       {"fixup_t2_pcrel_9", 0, 32,
-       MCFixupKindInfo::FKF_IsPCRel |
-           MCFixupKindInfo::FKF_IsAlignedDownTo32Bits},
+       IsPCRelConstant | MCFixupKindInfo::FKF_IsAlignedDownTo32Bits},
       {"fixup_thumb_adr_pcrel_10", 0, 8,
-       MCFixupKindInfo::FKF_IsPCRel |
-           MCFixupKindInfo::FKF_IsAlignedDownTo32Bits},
-      {"fixup_arm_adr_pcrel_12", 0, 32, MCFixupKindInfo::FKF_IsPCRel},
+       IsPCRelConstant | MCFixupKindInfo::FKF_IsAlignedDownTo32Bits},
+      {"fixup_arm_adr_pcrel_12", 0, 32, IsPCRelConstant},
       {"fixup_t2_adr_pcrel_12", 0, 32,
-       MCFixupKindInfo::FKF_IsPCRel |
-           MCFixupKindInfo::FKF_IsAlignedDownTo32Bits},
+       IsPCRelConstant | MCFixupKindInfo::FKF_IsAlignedDownTo32Bits},
       {"fixup_arm_condbranch", 0, 24, MCFixupKindInfo::FKF_IsPCRel},
       {"fixup_arm_uncondbranch", 0, 24, MCFixupKindInfo::FKF_IsPCRel},
       {"fixup_t2_condbranch", 0, 32, MCFixupKindInfo::FKF_IsPCRel},

test/MC/ARM/Windows/invalid-relocation.s

@@ -7,7 +7,6 @@
 	.endef
 	.global invalid_relocation
 	.thumb_func
-invalid_relocation:
 	adr r0, invalid_relocation+1
 
 # CHECK: LLVM ERROR: unsupported relocation type: fixup_t2_adr_pcrel_12

test/MC/ARM/pcrel-global.s

@@ -0,0 +1,21 @@
+@ RUN: llvm-mc -filetype=obj -triple=armv7 %s -o %t
+@ RUN: llvm-readelf -r %t | FileCheck %s
+
+@ CHECK: There are no relocations in this file.
+.syntax unified
+
+.globl foo
+foo:
+ldrd r0, r1, foo @ arm_pcrel_10_unscaled
+vldr d0, foo     @ arm_pcrel_10
+adr r2, foo      @ arm_adr_pcrel_12
+ldr r0, foo      @ arm_ldst_pcrel_12
+
+.thumb
+.thumb_func
+
+.globl bar
+bar:
+adr r0, bar      @ thumb_adr_pcrel_10
+adr.w r0, bar    @ t2_adr_pcrel_12
+ldr.w pc, bar    @ t2_ldst_pcrel_12

test/MC/MachO/ARM/bad-darwin-ARM-reloc.s

@@ -6,7 +6,6 @@
 .section myseg, mysect
 L___fcommon: 
     .word 0
-@ CHECK-ERROR: unsupported relocation on symbol
 
 c:
   .word a - b