[ELF][MIPS] Produce a correct and complete set of MIPS ELF header flags

[atanasyan]

Summary

The patch extends the getMipsEFlags function. Now in that function we iterate over all object files, parse ELF header flags and merge them. If a file is incompatible with previously analyzed ones we show an error or warning. That can happen if, for example, we try to link files with incompatible ABI, ISA, NAN encoding etc.

There is an alternative solution. We can check and merge flags and reject incompatible input modules in the isCompatible function which is called from the SymbolTable::addFile method. But in that case we have to save and keep somewhere a merged ELF flags combination to use it later in the writer.

[ruiu]

ELF/Writer.cpp

1242–1262 ↗

(On Diff #66791)

I cannot find a simple rule for this code. What is the expected output for what inputs? Is there a documentation to describe the rule behind it?

[atanasyan]

ELF/Writer.cpp

1242–1262 ↗

(On Diff #66791)

I cannot find a simple rule for this code. What is the expected output for what inputs? Is there a documentation to describe the rule behind it?

As usual there is no documentation on this code. The general idea - ELF flags define ISA (R_MIPS_32, R_MIPS_32R2, R_MIPS_64 ...), ABI (R_MIPS_O32, R_MIPS_ABI2 ...) and features flags (R_MIPS_NAN2008, R_MIPS_PIC ...). Some kinds of ISAs are compatible with each other and we need to select the "highest" from input ISAs. For example, if one input file has R_MIPS_32 ISA and another one has R_MIPS_32R2 ISA, we need to put R_MIPS_32R2 into the output file. SOme kinds of ISAs are incompatible. We should reject attempts to link R_MIPS_32R2 and R_MIPS_32R6 ISAs together. The same logic is applicable to ABI and some features flags like R_MIPS_NAN2008 and R_MIPS_FP64. Other features flags like EF_MIPS_NOREORDER, EF_MIPS_MICROMIPS etc can be just merged.

As to the R_MIPS_PIC and R_MIPS_CPIC flags - PIC code is inherently CPIC and may not set CPIC flag explicitly. But it is a good practice to set it in the output file so we do that. If we merge PIC and non-PIC code, output file cannot be considered as position independent so we clear PIC flag if as soon as we see non-PIC input file. If input file does not have PIC not CPIC flags that means that this file does not follow abicalls convention and we show a warning if linker mix abicalls and non-abicalls files.

[ruiu]

ELF/Writer.cpp

1184 ↗

(On Diff #66791)

This and the following two "tostring" functions seem a bit odd to be here in the linker. Don't you think they should actually live in LLVM instead of LLD?

1242–1262 ↗

(On Diff #66791)

Thank you for the explanation. So it seems to be inevitably complicated. Do you think you can split this function into small functions, each of which checks only one thing? I'd imagine we could have getMipsPicFlag(), getMipsArch(), getMipsAbi(), etc.

[atanasyan]

Thanks for review.

In this patch I moved a code from getMipsEFlags into some small functions to check and update ISA, ABI, PIC, and features flags separately.

I like the idea to move "tostring" function into LLVM code base. But I cannot find the right place for them. Any ideas?

[ruiu]

LGTM. As to moving the code to LLVM, I have no idea. But we can move the code later.

lld/trunk/ELF/Writer.cpp

for (Phdr &P : Phdrs) {
     if (H.p_type == PT_TLS) {
       Out<ELFT>::TlsPhdr = &H;
       if (H.p_memsz)
         H.p_memsz = alignTo(H.p_memsz, H.p_align);
     }
   }
 }
 
+namespace {
+struct MipsIsaTreeEdge {
+  uint32_t Child;
+  uint32_t Parent;
+};
+}
+
+static MipsIsaTreeEdge MipsIsaTree[] = {
+    // MIPS32R6 and MIPS64R6 are not compatible with other extensions
+    // MIPS64 extensions.
+    {EF_MIPS_ARCH_64R2, EF_MIPS_ARCH_64},
+    // MIPS V extensions.
+    {EF_MIPS_ARCH_64, EF_MIPS_ARCH_5},
+    // MIPS IV extensions.
+    {EF_MIPS_ARCH_5, EF_MIPS_ARCH_4},
+    // MIPS III extensions.
+    {EF_MIPS_ARCH_4, EF_MIPS_ARCH_3},
+    // MIPS32 extensions.
+    {EF_MIPS_ARCH_32R2, EF_MIPS_ARCH_32},
+    // MIPS II extensions.
+    {EF_MIPS_ARCH_3, EF_MIPS_ARCH_2},
+    {EF_MIPS_ARCH_32, EF_MIPS_ARCH_2},
+    // MIPS I extensions.
+    {EF_MIPS_ARCH_2, EF_MIPS_ARCH_1},
+};
+
+static bool isMipsIsaMatched(uint32_t New, uint32_t Res) {
+  if (New == Res)
+    return true;
+  if (New == EF_MIPS_ARCH_32 && isMipsIsaMatched(EF_MIPS_ARCH_64, Res))
+    return true;
+  if (New == EF_MIPS_ARCH_32R2 && isMipsIsaMatched(EF_MIPS_ARCH_64R2, Res))
+    return true;
+  for (const auto &Edge : MipsIsaTree) {
+    if (Res == Edge.Child) {
+      Res = Edge.Parent;
+      if (Res == New)
+        return true;
+    }
+  }
+  return false;
+}
+
+static StringRef getMipsIsaName(uint32_t Flags) {
+  switch (Flags) {
+  case EF_MIPS_ARCH_1:
+    return "mips1";
+  case EF_MIPS_ARCH_2:
+    return "mips2";
+  case EF_MIPS_ARCH_3:
+    return "mips3";
+  case EF_MIPS_ARCH_4:
+    return "mips4";
+  case EF_MIPS_ARCH_5:
+    return "mips5";
+  case EF_MIPS_ARCH_32:
+    return "mips32";
+  case EF_MIPS_ARCH_64:
+    return "mips64";
+  case EF_MIPS_ARCH_32R2:
+    return "mips32r2";
+  case EF_MIPS_ARCH_64R2:
+    return "mips64r2";
+  case EF_MIPS_ARCH_32R6:
+    return "mips32r6";
+  case EF_MIPS_ARCH_64R6:
+    return "mips64r6";
+  default:
+    return "unknown";
+  }
+}
+
+static StringRef getMipsAbiName(uint32_t Flags) {
+  switch (Flags) {
+  case 0:
+    return "n64";
+  case EF_MIPS_ABI2:
+    return "n32";
+  case EF_MIPS_ABI_O32:
+    return "o32";
+  case EF_MIPS_ABI_O64:
+    return "o64";
+  case EF_MIPS_ABI_EABI32:
+    return "eabi32";
+  case EF_MIPS_ABI_EABI64:
+    return "eabi64";
+  default:
+    return "unknown";
+  }
+}
+
+static StringRef getMipsNanName(bool IsNan2008) {
+  return IsNan2008 ? "2008" : "legacy";
+}
+
+static StringRef getMipsFpName(bool IsFp64) { return IsFp64 ? "64" : "32"; }
+
+static uint32_t updateMipsPicFlags(uint32_t ResFlags, uint32_t NewFlags,
+                                   StringRef FName) {
+  uint32_t NewPic = NewFlags & (EF_MIPS_PIC | EF_MIPS_CPIC);
+  uint32_t ResPic = ResFlags & (EF_MIPS_PIC | EF_MIPS_CPIC);
+
+  // Check PIC / CPIC flags compatibility.
+  if (NewPic && !ResPic)
+    warning("linking non-abicalls code with abicalls file: " + FName);
+  if (!NewPic && ResPic)
+    warning("linking abicalls code with non-abicalls file: " + FName);
+
+  if (!(NewPic & EF_MIPS_PIC))
+    ResFlags &= ~EF_MIPS_PIC;
+  if (NewPic)
+    ResFlags |= EF_MIPS_CPIC;
+  return ResFlags;
+}
+
+static uint32_t updateMipsIsaFlags(uint32_t ResFlags, uint32_t NewFlags,
+                                   StringRef FName) {
+  uint32_t NewIsa = NewFlags & EF_MIPS_ARCH;
+  uint32_t ResIsa = ResFlags & EF_MIPS_ARCH;
+
+  // Check ISA compatibility.
+  if (isMipsIsaMatched(NewIsa, ResIsa))
+    return ResFlags;
+  if (!isMipsIsaMatched(ResIsa, NewIsa)) {
+    error("target isa '" + getMipsIsaName(ResIsa) + "' is incompatible with '" +
+          getMipsIsaName(NewIsa) + "': " + FName);
+    return ResFlags;
+  }
+  ResFlags &= ~EF_MIPS_ARCH;
+  ResFlags |= NewIsa;
+  return ResFlags;
+}
+
+static void checkMipsAbiFlags(uint32_t ResFlags, uint32_t NewFlags,
+                              StringRef FName) {
+  uint32_t NewAbi = NewFlags & (EF_MIPS_ABI | EF_MIPS_ABI2);
+  uint32_t ResAbi = ResFlags & (EF_MIPS_ABI | EF_MIPS_ABI2);
+  // Check ABI compatibility.
+  if (NewAbi != ResAbi)
+    error("target ABI '" + getMipsAbiName(ResAbi) + "' is incompatible with '" +
+          getMipsAbiName(NewAbi) + "': " + FName);
+}
+
+static void checkMipsNanFlags(uint32_t ResFlags, uint32_t NewFlags,
+                              StringRef FName) {
+  bool NewNan2008 = NewFlags & EF_MIPS_NAN2008;
+  bool ResNan2008 = ResFlags & EF_MIPS_NAN2008;
+  // Check -mnan flags compatibility.
+  if (NewNan2008 != ResNan2008)
+    error("target -mnan=" + getMipsNanName(ResNan2008) +
+          " is incompatible with -mnan=" + getMipsNanName(NewNan2008) + ": " +
+          FName);
+}
+
+static void checkMipsFpFlags(uint32_t ResFlags, uint32_t NewFlags,
+                             StringRef FName) {
+  bool NewFp64 = NewFlags & EF_MIPS_FP64;
+  bool ResFp64 = ResFlags & EF_MIPS_FP64;
+  // Check FP64 compatibility.
+  if (NewFp64 != ResFp64)
+    error("target -mfp" + getMipsFpName(ResFp64) +
+          " is incompatible with -mfp" + getMipsFpName(NewFp64) + ": " + FName);
+}
+
 template <class ELFT> static uint32_t getMipsEFlags() {
-  // FIXME: ELF flags depends on ELF flags of all input object files and
-  // selected emulation. For now pick the arch flag from the fisrt input file
-  // and use hard coded values for other flags.
-  uint32_t FirstElfFlags =
-      cast<ELFFileBase<ELFT>>(Config->FirstElf)->getObj().getHeader()->e_flags;
-  uint32_t ElfFlags = FirstElfFlags & EF_MIPS_ARCH;
-  if (ELFT::Is64Bits)
-    ElfFlags |= EF_MIPS_CPIC | EF_MIPS_PIC;
-  else {
-    ElfFlags |= EF_MIPS_CPIC | EF_MIPS_ABI_O32;
-    if (Config->Shared)
-      ElfFlags |= EF_MIPS_PIC;
+  // Iterates over all object files andretrieve ELF header flags
+  // to check that ISA, ABI and features declared by these flags
+  // are compatible with each other.
+  uint32_t ResFlags = 0;
+  for (const std::unique_ptr<elf::ObjectFile<ELFT>> &F :
+       Symtab<ELFT>::X->getObjectFiles()) {
+    uint32_t NewFlags = F->getObj().getHeader()->e_flags;
+    if (ResFlags == 0) {
+      if (NewFlags & EF_MIPS_PIC)
+        // PIC code is inherently CPIC
+        // and may not set CPIC flag explicitly.
+        NewFlags |= EF_MIPS_CPIC;
+      ResFlags = NewFlags;
+      continue;
+    }
+
+    ResFlags = updateMipsPicFlags(ResFlags, NewFlags, F->getName());
+    ResFlags = updateMipsIsaFlags(ResFlags, NewFlags, F->getName());
+
+    checkMipsAbiFlags(ResFlags, NewFlags, F->getName());
+    checkMipsNanFlags(ResFlags, NewFlags, F->getName());
+    checkMipsFpFlags(ResFlags, NewFlags, F->getName());
+
+    ResFlags |= NewFlags & EF_MIPS_ARCH_ASE;
+    ResFlags |= NewFlags & EF_MIPS_NOREORDER;
+    ResFlags |= NewFlags & EF_MIPS_MICROMIPS;
+    ResFlags |= NewFlags & EF_MIPS_NAN2008;
+    ResFlags |= NewFlags & EF_MIPS_32BITMODE;
   }
-  return ElfFlags;
+  return ResFlags;
 }
 
 template <class ELFT> static typename ELFT::uint getEntryAddr() {
   if (Symbol *S = Config->EntrySym)
     return S->body()->getVA<ELFT>();
   if (Config->EntryAddr != uint64_t(-1))
     return Config->EntryAddr;
   return 0;

lld/trunk/test/ELF/mips-elf-flags-err.s

+# Check MIPS ELF ISA flag calculation if input files have different ISAs.
+
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux \
+# RUN:         -mcpu=mips32 %S/Inputs/mips-dynamic.s -o %t1.o
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux \
+# RUN:         -mcpu=mips32r2 %s -o %t2.o
+# RUN: ld.lld %t1.o %t2.o -o %t.exe
+# RUN: llvm-readobj -h %t.exe | FileCheck -check-prefix=R1R2 %s
+
+# Check that lld does not allow to link incompatible ISAs.
+
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux \
+# RUN:         -mcpu=mips32 %S/Inputs/mips-dynamic.s -o %t1.o
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux \
+# RUN:         -mcpu=mips32r6 %s -o %t2.o
+# RUN: not ld.lld %t1.o %t2.o -o %t.exe 2>&1 | FileCheck -check-prefix=R1R6 %s
+
+# Check that lld does not allow to link incompatible ABIs.
+
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux \
+# RUN:         -target-abi n32 %S/Inputs/mips-dynamic.s -o %t1.o
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux \
+# RUN:         -target-abi o32 %s -o %t2.o
+# RUN: not ld.lld %t1.o %t2.o -o %t.exe 2>&1 | FileCheck -check-prefix=N32O32 %s
+
+# Check that lld does not allow to link modules with incompatible NAN flags.
+
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux \
+# RUN:         -mattr=+nan2008 %S/Inputs/mips-dynamic.s -o %t1.o
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux \
+# RUN:         %s -o %t2.o
+# RUN: not ld.lld %t1.o %t2.o -o %t.exe 2>&1 | FileCheck -check-prefix=NAN %s
+
+# REQUIRES: mips
+
+  .option pic0
+  .text
+  .global  __start
+__start:
+  nop
+
+# R1R2:      Flags [
+# R1R2-NEXT:   EF_MIPS_ABI_O32
+# R1R2-NEXT:   EF_MIPS_ARCH_32R2
+# R1R2-NEXT:   EF_MIPS_CPIC
+# R1R2-NEXT: ]
+
+# R1R6: target isa 'mips32' is incompatible with 'mips32r6': {{.*}}mips-elf-flags-err.s.tmp2.o
+
+# N32O32: target ABI 'n32' is incompatible with 'o32': {{.*}}mips-elf-flags-err.s.tmp2.o
+
+# NAN: target -mnan=2008 is incompatible with -mnan=legacy: {{.*}}mips-elf-flags-err.s.tmp2.o

lld/trunk/test/ELF/mips-elf-flags.s

 # Check generation of MIPS specific ELF header flags.
 
-# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t.o
-# RUN: ld.lld %t.o -shared -o %t.so
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux \
+# RUN:         %S/Inputs/mips-dynamic.s -o %t-so.o
+# RUN: ld.lld %t-so.o -shared -o %t.so
 # RUN: llvm-readobj -h %t.so | FileCheck -check-prefix=SO %s
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t.o
 # RUN: ld.lld %t.o -o %t.exe
 # RUN: llvm-readobj -h %t.exe | FileCheck -check-prefix=EXE %s
 # RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux \
 # RUN:         -mcpu=mips32r2 %s -o %t-r2.o
 # RUN: ld.lld %t-r2.o -o %t-r2.exe
 # RUN: llvm-readobj -h %t-r2.exe | FileCheck -check-prefix=EXE-R2 %s
 # RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux \
 # RUN:         -mcpu=mips32r6 %s -o %t-r6.o

 # EXE-R2-NEXT:   EF_MIPS_ARCH_32R2
 # EXE-R2-NEXT:   EF_MIPS_CPIC
 # EXE-R2-NEXT: ]
 
 # EXE-R6:      Flags [
 # EXE-R6-NEXT:   EF_MIPS_ABI_O32
 # EXE-R6-NEXT:   EF_MIPS_ARCH_32R6
 # EXE-R6-NEXT:   EF_MIPS_CPIC
+# EXE-R6-NEXT:   EF_MIPS_NAN2008
 # EXE-R6-NEXT: ]