[ELF][ARM] Implement support for Tag_ABI_VFP_args

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.
    
References:
- Addenda to, and Errata in, the ABI for the ARM Architecture for
Tag_ABI_VFP_args
- Elf for the ARM Architecture for ELF header flags
    
Fixes PR36009
    
Differential Revision: https://reviews.llvm.org/D49993

llvm-svn: 338377

1 files changed, 42 insertions, 0 deletions

diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
index 6da722f6f30e..0eb605a556ae 100644
--- a/lld/ELF/InputFiles.cpp
+++ b/lld/ELF/InputFiles.cpp
@@ -494,6 +494,46 @@ void ObjFile<ELFT>::initializeSections(
   }
 }
 
+// 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.
@@ -573,6 +613,8 @@ InputSectionBase *ObjFile<ELFT>::createInputSection(const Elf_Shdr &Sec) {
     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.