[MC][ARM] Emit R_ARM_BASE_PREL for _GLOBAL_OFFSET_TABLE_ expressions

[peter.smith]

Summary

The _GLOBAL_OFFSET_TABLE_ in SysVr4 ELF is conventionally the base of the .got or .got.prel sections. Expressions such as _GLOBAL_OFFSET_TABLE - (.L1+8) are used
in assembler code to calculate offsets into the .got. At present MC outputs a R_ARM_REL32 with respect to the _GLOBAL_OFFSET_TABLE_ symbol, whereas gas outputs a
R_ARM_BASE_PREL relocation with respect to the _GLOBAL_OFFSET_TABLE_ symbol. While both are correct, the R_ARM_REL32 depends on the value of the
_GLOBAL_OFFSET_TABLE_ symbol, whereas the R_ARM_BASE_PREL relocation is independent of the symbol (it resolves to the base of whatever the linker defines the base of the .got to be). The R_ARM_BASE_PREL is therefore slightly more robust to linker's that may not follow the convention of placement of the _GLOBAL_OFFSET_TABLE_; for example LLD for some time defined _GLOBAL_OFFSET_TABLE_ to 0.

Fixes PR33511 (https://bugs.llvm.org/show_bug.cgi?id=33511)

There are a number of other backends (Sparc, X86 and PPC) that special case _GLOBAL_OFFSET_TABLE_ although all need to do it for different cases. The GNU as conversion can be found in tc-arm.c (search for BFD_RELOC_ARMGOTPC), gas also substitutes R_ARM_BASE_PREL for absolute expressions involving _GLOBAL_OFFSET_TABLE_, I'm not sure that this is correct as R_ARM_BASE_PREL is a relative relocation so I've not followed that behaviour.

Arm relocations defined in http://infocenter.arm.com/help/topic/com.arm.doc.ihi0044f/IHI0044F_aaelf.pdf

[MaskRay]

This patch still applies. This is similar to D47507 (x86).

The R_ARM_BASE_PREL is therefore slightly more robust to linker's that may not follow the convention of placement of the _GLOBAL_OFFSET_TABLE_; for example LLD for some time defined _GLOBAL_OFFSET_TABLE_ to 0.

Agree that this is slightly more robust. We may not need to worry much about the cases when _GLOBAL_OFFSET_TABLE_ is missing in the symbol table, because glibc crti.o (sysdeps/arm/crti.S) references it:

.word _GLOBAL_OFFSET_TABLE_-(.LPIC+8)

[psmith]

Thanks for the review, will retest and commit tomorrow (public holiday in UK today)

llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp

if (IsPCRel) {
     switch (Fixup.getTargetKind()) {
     default:
       Ctx.reportFatalError(Fixup.getLoc(), "unsupported relocation on symbol");
       return ELF::R_ARM_NONE;
     case FK_Data_4:
       switch (Modifier) {
       default:
         llvm_unreachable("Unsupported Modifier");
-      case MCSymbolRefExpr::VK_None:
+      case MCSymbolRefExpr::VK_None: {
+        if (const MCSymbolRefExpr *SymRef = Target.getSymA()) {
+          // For GNU AS compatibility expressions such as
+          // _GLOBAL_OFFSET_TABLE_ - label emit a R_ARM_BASE_PREL relocation.
+          if (SymRef->getSymbol().getName() == "_GLOBAL_OFFSET_TABLE_")
+            return ELF::R_ARM_BASE_PREL;
+        }
         return ELF::R_ARM_REL32;
+      }
       case MCSymbolRefExpr::VK_GOTTPOFF:
         return ELF::R_ARM_TLS_IE32;
       case MCSymbolRefExpr::VK_ARM_GOT_PREL:
         return ELF::R_ARM_GOT_PREL;
       case MCSymbolRefExpr::VK_ARM_PREL31:
         return ELF::R_ARM_PREL31;
       }
     case ARM::fixup_arm_blx:

llvm/test/MC/ARM/symbol-variants.s

 @CHECK: 6c R_ARM_PREL31 f27
 
 @ tlsldm
 .word f28(TLSLDM)
 .word f29(tlsldm)
 @CHECK: 70 R_ARM_TLS_LDM32 f28
 @CHECK: 74 R_ARM_TLS_LDM32 f29
 
+@ relative
+.word f30 - (.Lsym+8)
+@CHECK: 78 R_ARM_REL32 f30
+
+@ _GLOBAL_OFFSET_TABLE_ relative
+.word _GLOBAL_OFFSET_TABLE_ - (.Lsym+8)
+@CHECK: 7c R_ARM_BASE_PREL _GLOBAL_OFFSET_TABLE_
+
 @ got_prel
-.word  f30(GOT_PREL) + (. - .Lsym)
-  ldr r3, =f31(GOT_PREL)
-@ CHECK: 78 R_ARM_GOT_PREL f30
-@ CHECK: 80 R_ARM_GOT_PREL f31
+.word   f31(GOT_PREL) + (. - .Lsym)
+        ldr r3, =f32(GOT_PREL)
+@CHECK: 80 R_ARM_GOT_PREL f31
+@CHECK: 88 R_ARM_GOT_PREL f32