[LLD][ELF][ARM] Implement support for Tag_ABI_VFP_args

[peter.smith]

Summary

The Tag_ABI_VFP_args build attribute controls the procedure call standard used for floating point parameters on ARM. The values are:

0 - Base AAPCS (FP Parameters passed in Core (Integer) registers
1 - VFP AAPCS (FP Parameters passed in FP registers)
2 - Toolchain specific (Neither Base or VFP)
3 - Compatible with all (No use of floating point parameters)

If the Tag_ABI_VFP_args build attribute is missing it has an implicit value of 0.

We use the attribute in two ways:

• Detect a clash in calling convention between Base, VFP and Toolchain. We follow ld.bfd's lead and do not error if there is a clash between an implicit Base AAPCS caused by a missing attribute. Many projects including the hard-float (VFP AAPCS) version of glibc contain assembler files that do not use floating point but do not have Tag_ABI_VFP_args.
• Set the EF_ARM_ABI_FLOAT_SOFT or EF_ARM_ABI_FLOAT_HARD ELF header flag for Base or VFP AAPCS respectively. This flag is used by some ELF loaders.

The patch is based on the patch posted by Mark Kettenis to llvm-dev http://lists.llvm.org/pipermail/llvm-dev/2018-July/124876.html . It is dependent on D49992 to add the EF_ARM_ABI_FLOAT_SOFT and EF_ARM_ABI_FLOAT_HARD ELF header flags.

References:

• Addenda to, and Errata in, the ABI for the ARM Architecture for Tag_ABI_VFP_args http://infocenter.arm.com/help/topic/com.arm.doc.ihi0045e/IHI0045E_ABI_addenda.pdf
• Elf for the ARM Architecture for ELF header flags http://infocenter.arm.com/help/topic/com.arm.doc.ihi0044f/IHI0044F_aaelf.pdf

Fixes PR36009 (https://bugs.llvm.org/show_bug.cgi?id=36009)

[kettenis]

LGTM

Nothing significant from my original diff survives, so as far as copyright is concerned, this can totally be considered to be Peter's work.

[ruiu]

Overall looking good.

ELF/InputFiles.cpp

497

I'd start this comment with "This is ARM only" to make it clear that this is ARM only.

512

nit: since you are using this local variable only once, you can inline it.

519

If we don't have an enum variable for this magic variable, should we add that one to LLVM?

[grimar]

Looks good to me too. Minor comments below.

ELF/InputFiles.cpp

501

You do not need the namespace name here (elf::) I think.

520

toolchain -> Toolchain

526

Should default (it's unknown, right?) value trigger an error/warning/unreachable instead of return?

test/ELF/Inputs/arm-vfp-arg-base.s

12

I would add a comment, just like in other inputs you have.

[peter.smith]

Thanks all for the comments.

I've updated the patch with as many of the review comments addressed as possible:

• D50049 raised to add the missing enum values for Tag_ABI_VFP_args
• I've made it an error if the Tag_ABI_VFP_args contains an out of range value (I've now used VFPArgs twice so I've not inlined it). I've added a test to check we catch the illegal value.
• Comments altered according to suggestions.

[grimar]

LGTM

ELF/Arch/ARM.cpp

ARM::ARM() {
   // J2 bits to be used to extend the branch range. On earlier Architectures
   // such as ARMv4, ARMv5 and ARMv6 (except ARMv6T2) the range is +/- 4MiB. If
   // support for the earlier encodings is added then when they are used the
   // ThunkSectionSpacing will need lowering.
   ThunkSectionSpacing = 0x1000000 - 0x30000;
 }
 
 uint32_t ARM::calcEFlags() const {
+  // The ABIFloatType is used by loaders to detect the floating point calling
+  // convention.
+  uint32_t ABIFloatType = 0;
+  if (Config->ARMVFPArgs == ARMVFPArgKind::Base ||
+      Config->ARMVFPArgs == ARMVFPArgKind::Default)
+    ABIFloatType = EF_ARM_ABI_FLOAT_SOFT;
+  else if (Config->ARMVFPArgs == ARMVFPArgKind::VFP)
+    ABIFloatType = EF_ARM_ABI_FLOAT_HARD;
+
   // We don't currently use any features incompatible with EF_ARM_EABI_VER5,
   // but we don't have any firm guarantees of conformance. Linux AArch64
   // kernels (as of 2016) require an EABI version to be set.
-  return EF_ARM_EABI_VER5;
+  return EF_ARM_EABI_VER5 | ABIFloatType;
 }
 
 RelExpr ARM::getRelExpr(RelType Type, const Symbol &S,
                         const uint8_t *Loc) const {
   switch (Type) {
   case R_ARM_THM_JUMP11:
     return R_PC;
   case R_ARM_CALL:

ELF/Config.h

 enum class OrphanHandlingPolicy { Place, Warn, Error };
 
 // For --sort-section and linkerscript sorting rules.
 enum class SortSectionPolicy { Default, None, Alignment, Name, Priority };
 
 // For --target2
 enum class Target2Policy { Abs, Rel, GotRel };
 
+// For tracking ARM Float Argument PCS
+enum class ARMVFPArgKind { Default, Base, VFP, ToolChain };
+
 struct SymbolVersion {
   llvm::StringRef Name;
   bool IsExternCpp;
   bool HasWildcard;
 };
 
 // This struct contains symbols version definition that
 // can be found in version script if it is used for link.
struct Configuration {
   bool ZWxneeded;
   DiscardPolicy Discard;
   ICFLevel ICF;
   OrphanHandlingPolicy OrphanHandling;
   SortSectionPolicy SortSection;
   StripPolicy Strip;
   UnresolvedPolicy UnresolvedSymbols;
   Target2Policy Target2;
+  ARMVFPArgKind ARMVFPArgs = ARMVFPArgKind::Default;
   BuildIdKind BuildId = BuildIdKind::None;
   ELFKind EKind = ELFNoneKind;
   uint16_t DefaultSymbolVersion = llvm::ELF::VER_NDX_GLOBAL;
   uint16_t EMachine = llvm::ELF::EM_NONE;
   llvm::Optional<uint64_t> ImageBase;
   uint64_t MaxPageSize;
   uint64_t MipsGotSize;
   uint64_t ZStackSize;

ELF/InputFiles.cpp

if (Sec.sh_flags & SHF_LINK_ORDER) {
         error("a section " + IS->Name +
               " with SHF_LINK_ORDER should not refer a non-regular "
               "section: " +
               toString(LinkSec));
     }
   }
 }
 
+// For ARM only, to set the EF_ARM_ABI_FLOAT_SOFT or EF_ARM_ABI_FLOAT_HARD
+// flag in the ELF Header we need to look at Tag_ABI_VFP_args to find out how
+// the input objects have been compiled.
+static void updateARMVFPArgs(const ARMAttributeParser &Attributes,
+                             const InputFile *F) {
+  if (!Attributes.hasAttribute(ARMBuildAttrs::ABI_VFP_args))
+    // If an ABI tag isn't present then it is implicitly given the value of 0
+    // which maps to ARMBuildAttrs::BaseAAPCS. However many assembler files,
+    // including some in glibc that don't use FP args (and should have value 3)
+    // don't have the attribute so we do not consider an implicit value of 0
+    // as a clash.
+    return;
+
+  unsigned VFPArgs = Attributes.getAttributeValue(ARMBuildAttrs::ABI_VFP_args);
+  ARMVFPArgKind Arg;
+  switch (VFPArgs) {
+  case ARMBuildAttrs::BaseAAPCS:
+    Arg = ARMVFPArgKind::Base;
+    break;
+  case ARMBuildAttrs::HardFPAAPCS:
+    Arg = ARMVFPArgKind::VFP;
+    break;
+  case ARMBuildAttrs::ToolChainFPPCS:
+    // Tool chain specific convention that conforms to neither AAPCS variant.
+    Arg = ARMVFPArgKind::ToolChain;
+    break;
+  case ARMBuildAttrs::CompatibleFPAAPCS:
+    // Object compatible with all conventions.
+    return;
+  default:
+    error(toString(F) + ": unknown Tag_ABI_VFP_args value: " + Twine(VFPArgs));
+    return;
+  }
+  // Follow ld.bfd and error if there is a mix of calling conventions.
+  if (Config->ARMVFPArgs != Arg && Config->ARMVFPArgs != ARMVFPArgKind::Default)
+    error(toString(F) + ": incompatible Tag_ABI_VFP_args");
+  else
+    Config->ARMVFPArgs = Arg;
+}
+
 // The ARM support in lld makes some use of instructions that are not available
 // on all ARM architectures. Namely:
 // - Use of BLX instruction for interworking between ARM and Thumb state.
 // - Use of the extended Thumb branch encoding in relocation.
 // - Use of the MOVT/MOVW instructions in Thumb Thunks.
 // The ARM Attributes section contains information about the architecture chosen
 // at compile time. We follow the convention that if at least one input object
 // is compiled with an architecture that supports these features then lld is
InputSectionBase *ObjFile<ELFT>::createInputSection(const Elf_Shdr &Sec) {
   switch (Sec.sh_type) {
   case SHT_ARM_ATTRIBUTES: {
     if (Config->EMachine != EM_ARM)
       break;
     ARMAttributeParser Attributes;
     ArrayRef<uint8_t> Contents = check(this->getObj().getSectionContents(&Sec));
     Attributes.Parse(Contents, /*isLittle*/ Config->EKind == ELF32LEKind);
     updateSupportedARMFeatures(Attributes);
+    updateARMVFPArgs(Attributes, this);
+
     // FIXME: Retain the first attribute section we see. The eglibc ARM
     // dynamic loaders require the presence of an attribute section for dlopen
     // to work. In a full implementation we would merge all attribute sections.
     if (InX::ARMAttributes == nullptr) {
       InX::ARMAttributes = make<InputSection>(*this, Sec, Name);
       return InX::ARMAttributes;
     }
     return &InputSection::Discarded;

test/ELF/Inputs/arm-vfp-arg-base.s

+  .arch armv7-a
+  .eabi_attribute 20, 1
+  .eabi_attribute 21, 1
+  .eabi_attribute 23, 3
+  .eabi_attribute 24, 1
+  .eabi_attribute 25, 1
+  .eabi_attribute 26, 2
+  .eabi_attribute 30, 6
+  .eabi_attribute 34, 1
+  .eabi_attribute 18, 4
+        .eabi_attribute 28, 0 // Tag_ABI_VFP_args = 0 (AAPCS, Base variant)
+
+        .syntax unified
+        .global f0
+        .type f0, %function
+f0:     bx lr

test/ELF/Inputs/arm-vfp-arg-compat.s

+  .arch armv7-a
+  .eabi_attribute 20, 1
+  .eabi_attribute 21, 1
+  .eabi_attribute 23, 3
+  .eabi_attribute 24, 1
+  .eabi_attribute 25, 1
+  .eabi_attribute 26, 2
+  .eabi_attribute 30, 6
+  .eabi_attribute 34, 1
+  .eabi_attribute 18, 4
+        .eabi_attribute 28, 3 // Tag_ABI_VFP_args = 3 (Compatible with all)
+
+        .syntax unified
+        .global f3
+        .type f3, %function
+f3:     bx lr

test/ELF/Inputs/arm-vfp-arg-toolchain.s

+  .arch armv7-a
+  .eabi_attribute 20, 1
+  .eabi_attribute 21, 1
+  .eabi_attribute 23, 3
+  .eabi_attribute 24, 1
+  .eabi_attribute 25, 1
+  .eabi_attribute 26, 2
+  .eabi_attribute 30, 6
+  .eabi_attribute 34, 1
+  .eabi_attribute 18, 4
+  .eabi_attribute 28, 2 // Tag_ABI_VFP_args = 2 (Toolchain specific)
+        .syntax unified
+        .global f2
+        .type f1, %function
+f2:     bx lr

test/ELF/Inputs/arm-vfp-arg-vfp.s

+  .arch armv7-a
+  .eabi_attribute 20, 1
+  .eabi_attribute 21, 1
+  .eabi_attribute 23, 3
+  .eabi_attribute 24, 1
+  .eabi_attribute 25, 1
+  .eabi_attribute 26, 2
+  .eabi_attribute 30, 6
+  .eabi_attribute 34, 1
+  .eabi_attribute 18, 4
+  .eabi_attribute 28, 1 // Tag_ABI_VFP_args = 1 (AAPCS, VFP variant)
+        .syntax unified
+        .global f1
+        .type f1, %function
+f1:     bx lr

test/ELF/arm-eabi-version.s

 // REQUIRES: arm
 // RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t.o
 // RUN: ld.lld -static %t.o -o %tout
 // RUN: llvm-readobj -file-headers %tout | FileCheck %s
  .syntax unified
  .text
  .globl _start
 _start:
  bx lr
 
 // CHECK:  Flags [
+// CHECK-NEXT:    0x200        
 // CHECK-NEXT:    0x1000000
 // CHECK-NEXT:    0x4000000
 // CHECK-NEXT:  ]

test/ELF/arm-tag-vfp-args-errs.s

+// REQUIRES:arm
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-vfp-arg-base.s -o %tbase.o
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-vfp-arg-vfp.s -o %tvfp.o
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-vfp-arg-toolchain.s -o %ttoolchain.o
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t.o
+// RUN: not ld.lld %t.o %tbase.o %tvfp.o 2>&1 | FileCheck %s
+// RUN: not ld.lld %t.o %tbase.o %ttoolchain.o 2>&1 | FileCheck %s
+// RUN: not ld.lld %t.o %tvfp.o %tbase.o 2>&1 | FileCheck %s
+// RUN: not ld.lld %t.o %tvfp.o %ttoolchain.o 2>&1 | FileCheck %s
+// RUN: not ld.lld %t.o %ttoolchain.o %tbase.o 2>&1 | FileCheck %s
+// RUN: not ld.lld %t.o %ttoolchain.o %tvfp.o 2>&1 | FileCheck %s
+
+// CHECK: incompatible Tag_ABI_VFP_args
+  .arch armv7-a
+  .eabi_attribute 20, 1
+  .eabi_attribute 21, 1
+  .eabi_attribute 23, 3
+  .eabi_attribute 24, 1
+  .eabi_attribute 25, 1
+  .eabi_attribute 26, 2
+  .eabi_attribute 30, 6
+  .eabi_attribute 34, 1
+  .eabi_attribute 18, 4
+        .eabi_attribute 28, 3 // Tag_ABI_VFP_args = 3 (Compatible with all)
+
+        .syntax unified
+        .globl _start
+        .type _start, %function
+_start: bx lr

test/ELF/arm-tag-vfp-args-illegal.s

+// REQUIRES:arm
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t.o
+// RUN: not ld.lld %t.o -o %t 2>&1 | FileCheck %s
+
+// CHECK: arm-tag-vfp-args-illegal.s.tmp.o: unknown Tag_ABI_VFP_args value: 5
+        .arch armv7-a
+        .eabi_attribute 20, 1
+        .eabi_attribute 21, 1
+        .eabi_attribute 23, 3
+        .eabi_attribute 24, 1
+        .eabi_attribute 25, 1
+        .eabi_attribute 26, 2
+        .eabi_attribute 30, 6
+        .eabi_attribute 34, 1
+        .eabi_attribute 18, 4
+        .eabi_attribute 28, 5 // Tag_ABI_VFP_args = 5 (Illegal value)
+
+        .syntax unified
+        .globl _start
+        .type _start, %function
+_start: bx lr

test/ELF/arm-tag-vfp-args.s

+// REQUIRES:arm
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-vfp-arg-base.s -o %tbase.o
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-vfp-arg-vfp.s -o %tvfp.o
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-vfp-arg-toolchain.s -o %ttoolchain.o
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-vfp-arg-compat.s -o %tcompat.o
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t.o
+
+// The default for this file is 0 (Base AAPCS)
+// RUN: ld.lld %t.o -o %tdefault
+// RUN: llvm-readobj -file-headers %tdefault | FileCheck -check-prefix=CHECK-BASE %s
+
+// Expect explicit Base AAPCS.
+// RUN: ld.lld %t.o %tbase.o -o %tbase
+// RUN: llvm-readobj -file-headers %tbase | FileCheck -check-prefix=CHECK-BASE %s
+
+// Expect explicit Base AAPCS when linking Base and Compatible.
+// RUN: ld.lld %t.o %tbase.o %tcompat.o -o %tbasecompat
+// RUN: llvm-readobj -file-headers %tbasecompat | FileCheck -check-prefix=CHECK-BASE %s
+
+// CHECK-BASE:   Flags [ (0x5000200)
+// CHECK-BASE-NEXT:     0x200
+// CHECK-BASE-NEXT:     0x1000000
+// CHECK-BASE-NEXT:     0x4000000
+// CHECK-BASE-NEXT:   ]
+
+// Expect Hard float VFP AAPCS
+// RUN: ld.lld %t.o %tvfp.o -o %tvfp
+// RUN: llvm-readobj -file-headers %tvfp | FileCheck -check-prefix=CHECK-VFP %s
+
+// Expect Hard float VFP AAPCS when linking VFP and Compatible
+// RUN: ld.lld %t.o %tvfp.o %tcompat.o -o %tvfpcompat
+// RUN: llvm-readobj -file-headers %tvfpcompat | FileCheck -check-prefix=CHECK-VFP %s
+
+// CHECK-VFP:   Flags [ (0x5000400)
+// CHECK-VFP-NEXT:     0x400
+// CHECK-VFP-NEXT:     0x1000000
+// CHECK-VFP-NEXT:     0x4000000
+// CHECK-VFP-NEXT:   ]
+
+// Expect Toolchain specifc to not use either Base or VFP AAPCS
+// RUN: ld.lld %t.o %ttoolchain.o -o %ttoolchain
+// RUN: llvm-readobj -file-headers %ttoolchain | FileCheck -check-prefix=CHECK-TOOLCHAIN %s
+
+// Expect Toolchain and Compatible to have same affect as Toolchain.
+// RUN: ld.lld %t.o %ttoolchain.o %tcompat.o -o %ttoolchaincompat
+// RUN: llvm-readobj -file-headers %ttoolchaincompat | FileCheck -check-prefix=CHECK-TOOLCHAIN %s
+
+// CHECK-TOOLCHAIN:   Flags [ (0x5000000)
+// CHECK-TOOLCHAIN-NEXT:     0x1000000
+// CHECK-TOOLCHAIN-NEXT:     0x4000000
+// CHECK-TOOLCHAIN-NEXT:   ]
+
+        .arch armv7-a
+        .eabi_attribute 20, 1
+        .eabi_attribute 21, 1
+        .eabi_attribute 23, 3
+        .eabi_attribute 24, 1
+        .eabi_attribute 25, 1
+        .eabi_attribute 26, 2
+        .eabi_attribute 30, 6
+        .eabi_attribute 34, 1
+        .eabi_attribute 18, 4
+        // We do not specify Tag_ABI_VFP_args (.eabi_attribute 28) in this file.
+        // When omitted the value of the tag defaults to 0, however if there
+        // are other files with explicit Tag_ABI_VFP_args we use that in
+        // preference.
+
+
+        .syntax unified
+        .globl _start
+        .type _start, %function
+_start:  bx lr