[ELF][RISCV] Relax local-exec TLS model

[MaskRay]

Summary

In -mrelax mode, GCC/Clang may generate a local-exec TLS code sequence like:

# R_RISCV_TPREL_HI20, R_RISCV_RELAX
lui rd, %tprel_hi(x)
# R_RISCV_TPREL_ADD, R_RISCV_RELAX
add rd, rd, tp, %tprel_add(x)
# (R_RISCV_TPREL_LO12_I || R_RISCV_TPREL_LO12_S), R_RISCV_RELAX
addi rd, rd, %tprel_lo(x) || sw rs, %tprel(x)(rd)

Note: st_value(x) for TLS should be in the range [0,p_memsz(PT_TLS)).
When st_value(x) < 2048 (i.e. hi20(x) == 0), the linker can relax
the code sequence to:

addi rd, tp, st_value(x) || sw rs, st_value(x)(rd)

[MaskRay]

lld/ELF/Arch/RISCV.cpp

576

I'll remove the used-once sym

[jrtc27]

llvm/include/llvm/BinaryFormat/ELFRelocs/RISCV.def

49

These encodings are reserved for future standard use

[MaskRay]

avoid R_RISCV_TPREL_{I,S}

[MaskRay]

I'll push this on Friday.

[MaskRay]

Test non-adjacent instructions

lld/ELF/Arch/RISCV.cpp

enum Op {
   LD = 0x3003,
   LW = 0x2003,
   SRLI = 0x5013,
   SUB = 0x40000033,
 };
 
 enum Reg {
   X_RA = 1,
+  X_TP = 4,
   X_T0 = 5,
   X_T1 = 6,
   X_T2 = 7,
   X_T3 = 28,
 };
 
 static uint32_t hi20(uint32_t val) { return (val + 0x800) >> 12; }
 static uint32_t lo12(uint32_t val) { return val & 4095; }
case R_RISCV_TLS_GD_HI20:
     return R_TLSGD_PC;
   case R_RISCV_TLS_GOT_HI20:
     config->hasTlsIe = true;
     return R_GOT_PC;
   case R_RISCV_TPREL_HI20:
   case R_RISCV_TPREL_LO12_I:
   case R_RISCV_TPREL_LO12_S:
     return R_TPREL;
-  case R_RISCV_TPREL_ADD:
-    return R_NONE;
   case R_RISCV_ALIGN:
     return R_RELAX_HINT;
+  case R_RISCV_TPREL_ADD:
   case R_RISCV_RELAX:
     return config->relax ? R_RELAX_HINT : R_NONE;
   default:
     error(getErrorLocation(loc) + "unknown relocation (" + Twine(type) +
           ") against symbol " + toString(s));
     return R_NONE;
   }
 }
case R_RISCV_HI20: {
     uint64_t hi = val + 0x800;
     checkInt(loc, SignExtend64(hi, bits) >> 12, 20, rel);
     write32le(loc, (read32le(loc) & 0xFFF) | (hi & 0xFFFFF000));
     return;
   }
 
   case R_RISCV_PCREL_LO12_I:
   case R_RISCV_TPREL_LO12_I:
+  case R_RISCV_TPREL_I:
   case R_RISCV_LO12_I: {
     uint64_t hi = (val + 0x800) >> 12;
     uint64_t lo = val - (hi << 12);
     write32le(loc, (read32le(loc) & 0xFFFFF) | ((lo & 0xFFF) << 20));
     return;
   }
 
   case R_RISCV_PCREL_LO12_S:
   case R_RISCV_TPREL_LO12_S:
+  case R_RISCV_TPREL_S:
   case R_RISCV_LO12_S: {
     uint64_t hi = (val + 0x800) >> 12;
     uint64_t lo = val - (hi << 12);
     uint32_t imm11_5 = extractBits(lo, 11, 5) << 25;
     uint32_t imm4_0 = extractBits(lo, 4, 0) << 7;
     write32le(loc, (read32le(loc) & 0x1FFF07F) | imm11_5 | imm4_0);
     return;
   }
if (rvc && isInt<12>(displace) && rd == 0) {
     remove = 6;
   } else if (isInt<21>(displace)) {
     sec.relaxAux->relocTypes[i] = R_RISCV_JAL;
     sec.relaxAux->writes.push_back(0x6f | rd << 7); // jal
     remove = 4;
   }
 }
 
+// Relax local-exec TLS when hi20 is zero.
+static void relaxTlsLe(const InputSection &sec, size_t i, uint64_t loc,
+                      Relocation &r, uint32_t &remove) {
+  const Symbol &sym = *r.sym;
+  if (hi20(sym.getVA(r.addend)) != 0)
+    return;
+  const uint32_t insn = read32le(sec.rawData.data() + r.offset);
+  switch (r.type) {
+  case R_RISCV_TPREL_HI20:
+  case R_RISCV_TPREL_ADD:
+    // Remove lui rd, %tprel_hi(x) and add rd, rd, tp, %tprel_add(x).
+    sec.relaxAux->relocTypes[i] = R_RISCV_RELAX;
+    remove = 4;
+    break;
+  case R_RISCV_TPREL_LO12_I:
+    // addi rd, rd, %tprel_lo(x) => addi rd, tp, st_value(x)
+    sec.relaxAux->relocTypes[i] = R_RISCV_TPREL_I;
+    sec.relaxAux->writes.push_back((insn & ~(31 << 15)) | (X_TP << 15));
+    break;
+  case R_RISCV_TPREL_LO12_S:
+    // sw rs, %tprel_lo(x)(rd) => sw rs, st_value(x)(rd)
+    sec.relaxAux->relocTypes[i] = R_RISCV_TPREL_S;
+    sec.relaxAux->writes.push_back((insn & ~(31 << 15)) | (X_TP << 15));
+    break;
+  }
+}
+
 static bool relax(InputSection &sec) {
   const uint64_t secAddr = sec.getVA();
   auto &aux = *sec.relaxAux;
   bool changed = false;
 
   // Restore original st_value for symbols relative to this section.
   ArrayRef<SymbolAnchor> sa = makeArrayRef(aux.anchors);
   uint32_t delta = 0;
case R_RISCV_ALIGN: {
       break;
     }
     case R_RISCV_CALL:
     case R_RISCV_CALL_PLT:
       if (i + 1 != sec.relocations.size() &&
           sec.relocations[i + 1].type == R_RISCV_RELAX)
         relaxCall(sec, i, loc, r, remove);
       break;
+    case R_RISCV_TPREL_HI20:
+    case R_RISCV_TPREL_ADD:
+    case R_RISCV_TPREL_LO12_I:
+    case R_RISCV_TPREL_LO12_S:
+      if (i + 1 != sec.relocations.size() &&
+          sec.relocations[i + 1].type == R_RISCV_RELAX)
+        relaxTlsLe(sec, i, loc, r, remove);
+      break;
     }
 
     // For all anchors whose offsets are <= r.offset, they are preceded by
     // the previous relocation whose `relocDeltas` value equals `delta`.
     // Decrease their st_value and update their st_size.
     if (remove) {
       for (; sa.size() && sa[0].offset <= r.offset; sa = sa.slice(1)) {
         if (sa[0].end)
for (InputSection *sec : getInputSections(*osec, storage)) {
       sec->rawData = makeArrayRef(p, newSize);
       sec->bytesDropped = 0;
 
       // Update section content: remove NOPs for R_RISCV_ALIGN and rewrite
       // instructions for relaxed relocations.
       for (size_t i = 0, e = rels.size(); i != e; ++i) {
         uint32_t remove = aux.relocDeltas[i] - delta;
         delta = aux.relocDeltas[i];
-        if (remove == 0)
+        if (remove == 0 && aux.relocTypes[i] == R_RISCV_NONE)
           continue;
 
         // Copy from last location to the current relocated location.
         const Relocation &r = rels[i];
         uint64_t size = r.offset - offset;
         memcpy(p, old.data() + offset, size);
         p += size;
 
for (InputSection *sec : getInputSections(*osec, storage)) {
             for (; j + 4 <= skip; j += 4)
               write32le(p + j, 0x00000013); // nop
             if (j != skip) {
               assert(j + 2 == skip);
               write16le(p + j, 0x0001); // c.nop
             }
           }
         } else if (RelType newType = aux.relocTypes[i]) {
-          const uint32_t insn = aux.writes[writesIdx++];
           switch (newType) {
+          case R_RISCV_RELAX:
+            // Used by relaxTlsLe to indicate the relocation is ignored.
+            break;
           case R_RISCV_RVC_JUMP:
             skip = 2;
-            write16le(p, insn);
+            write16le(p, aux.writes[writesIdx++]);
             break;
           case R_RISCV_JAL:
+          case R_RISCV_TPREL_I:
+          case R_RISCV_TPREL_S:
             skip = 4;
-            write32le(p, insn);
+            write32le(p, aux.writes[writesIdx++]);
             break;
           default:
             llvm_unreachable("unsupported type");
           }
         }
 
         p += skip;
         offset = r.offset + skip + remove;

lld/test/ELF/riscv-tls-le.s

 # REQUIRES: riscv
 
+## Additionally test that (a) -no-pie/-pie have the same behavior
+## (b) --no-relax/--relax have the same behavior when R_RISCV_RELAX is suppressed.
 # RUN: llvm-mc -filetype=obj -triple=riscv32 %s -o %t.32.o
-# RUN: ld.lld %t.32.o -o %t.32
+# RUN: ld.lld --relax %t.32.o -o %t.32
 # RUN: llvm-nm -p %t.32 | FileCheck --check-prefixes=NM %s
 # RUN: llvm-objdump -d --no-show-raw-insn %t.32 | FileCheck --check-prefixes=LE %s
-# RUN: ld.lld -pie %t.32.o -o %t.32
+# RUN: ld.lld -pie --no-relax %t.32.o -o %t.32
 # RUN: llvm-objdump -d --no-show-raw-insn %t.32 | FileCheck --check-prefixes=LE %s
 
-# RUN: llvm-mc -filetype=obj -triple=riscv64 %s -o %t.64.o
-# RUN: ld.lld %t.64.o -o %t.64
+# RUN: llvm-mc -filetype=obj -triple=riscv64 -mattr=+relax %s -o %t.64.o
+# RUN: ld.lld --no-relax %t.64.o -o %t.64
 # RUN: llvm-objdump -d --no-show-raw-insn %t.64 | FileCheck --check-prefixes=LE %s
-# RUN: ld.lld -pie %t.64.o -o %t.64
+# RUN: ld.lld -pie --no-relax %t.64.o -o %t.64
 # RUN: llvm-objdump -d --no-show-raw-insn %t.64 | FileCheck --check-prefixes=LE %s
+# RUN: ld.lld %t.64.o -o %t.64.relax
+# RUN: llvm-objdump -d --no-show-raw-insn %t.64.relax | FileCheck --check-prefixes=LE-RELAX %s
 
 # RUN: not ld.lld -shared %t.32.o -o /dev/null 2>&1 | FileCheck %s --check-prefix=ERR --implicit-check-not=error:
 
 # ERR: error: relocation R_RISCV_TPREL_HI20 against .LANCHOR0 cannot be used with -shared
 # ERR: error: relocation R_RISCV_TPREL_LO12_I against .LANCHOR0 cannot be used with -shared
 # ERR: error: relocation R_RISCV_TPREL_HI20 against a cannot be used with -shared
 # ERR: error: relocation R_RISCV_TPREL_LO12_S against a cannot be used with -shared
+# ERR: error: relocation R_RISCV_TPREL_HI20 against a cannot be used with -shared
+# ERR: error: relocation R_RISCV_TPREL_LO12_S against a cannot be used with -shared
 
 # NM: {{0*}}00000008 b .LANCHOR0
-# NM: {{0*}}0000000c B a
+# NM: {{0*}}00000800 B a
 
 ## .LANCHOR0@tprel = 8
 ## a@tprel = 12
-# LE:      lui a5, 0
-# LE-NEXT: add a5, a5, tp
-# LE-NEXT: addi a5, a5, 8
-# LE-NEXT: lui a5, 0
+# LE:      lui a3, 0
+# LE-NEXT: add a3, a3, tp
+# LE-NEXT: addi a3, a3, 8
+# LE-NEXT: lui a4, 0
+# LE-NEXT: add a4, a4, tp
+# LE-NEXT: sw a0, 2044(a4)
+# LE-NEXT: lui a5, 1
 # LE-NEXT: add a5, a5, tp
-# LE-NEXT: sw a0, 12(a5)
+# LE-NEXT: sw a0, -2048(a5)
+# LE-EMPTY:
 
-lui a5, %tprel_hi(.LANCHOR0)
-add a5, a5, tp, %tprel_add(.LANCHOR0)
-addi a5, a5, %tprel_lo(.LANCHOR0)
+# LE-RELAX:      <.text>:
+# LE-RELAX-NEXT:   addi a3, tp, 8
+# LE-RELAX-NEXT:   sw a0, 2044(tp)
+# LE-RELAX-NEXT:   lui a5, 1
+# LE-RELAX-NEXT:   add a5, a5, tp
+# LE-RELAX-NEXT:   sw a0, -2048(a5)
+# LE-RELAX-EMPTY:
+
+lui a3, %tprel_hi(.LANCHOR0)
+add a3, a3, tp, %tprel_add(.LANCHOR0)
+addi a3, a3, %tprel_lo(.LANCHOR0)
+
+## hi20(a-4) = hi20(0x7fc) = 0. relaxable
+lui a4, %tprel_hi(a-4)
+add a4, a4, tp, %tprel_add(a-4)
+sw a0, %tprel_lo(a-4)(a4)
 
+## hi20(a) = hi20(0x800) = 1. not relaxable
 lui a5, %tprel_hi(a)
 add a5, a5, tp, %tprel_add(a)
 sw a0, %tprel_lo(a)(a5)
 
 .section .tbss
 .space 8
 .LANCHOR0:
-.zero 4
+.space 0x800-8
 .globl a
 a:
+.zero 4

llvm/include/llvm/BinaryFormat/ELFRelocs/RISCV.def

 ELF_RELOC(R_RISCV_SUB32,             39)
 ELF_RELOC(R_RISCV_SUB64,             40)
 ELF_RELOC(R_RISCV_GNU_VTINHERIT,     41)
 ELF_RELOC(R_RISCV_GNU_VTENTRY,       42)
 ELF_RELOC(R_RISCV_ALIGN,             43)
 ELF_RELOC(R_RISCV_RVC_BRANCH,        44)
 ELF_RELOC(R_RISCV_RVC_JUMP,          45)
 ELF_RELOC(R_RISCV_RVC_LUI,           46)
+ELF_RELOC(R_RISCV_TPREL_I,           49)
+ELF_RELOC(R_RISCV_TPREL_S,           50)
 ELF_RELOC(R_RISCV_RELAX,             51)
 ELF_RELOC(R_RISCV_SUB6,              52)
 ELF_RELOC(R_RISCV_SET6,              53)
 ELF_RELOC(R_RISCV_SET8,              54)
 ELF_RELOC(R_RISCV_SET16,             55)
 ELF_RELOC(R_RISCV_SET32,             56)
 ELF_RELOC(R_RISCV_32_PCREL,          57)
 ELF_RELOC(R_RISCV_IRELATIVE,         58)