[PPC64LE] write plt stubs for V2 abi.

[sfertile]

Summary

Adds the 'default' version of the plt stub for the V2 abi.

While all the instructions for this are correct, we are getting the wrong offset due to differences between power and the other architectures. lld puts the relocation for the function address into the .got.plt section, and the writePlt function gets passed the Offset within this section rather then the offset from the got which is what we would expect. The fix for this will be delivered in a subsequent patch.

[sfertile]

Updated path to remove the comment about why the offsets were different then I expected. It turns out that the weird offset is due to a few separate issues (ppc uses the sections differently, the sections are laid out in a different order, and that PPC64 target doesn't properly set the PltGot header size). The offsets will change as we fix those issues separately.

[ruiu]

LGTM

ELF/Arch/PPC64.cpp

177 ↗

(On Diff #139796)

Don't you think it's time to remove code for v1 API? It looks like we are doubling the size of the code unless we remove code for v1.

[sfertile]

ELF/Arch/PPC64.cpp

177 ↗

(On Diff #139796)

Yes, I have a couple patches waiting to upload, and I think we are probably ready to switch over (almost) all of the remaining tests to work with the V2 abi.

[ruiu]

That's good to know. Feel free to submit.

lld/trunk/ELF/Arch/PPC64.cpp

if (Config->EKind == ELF64LEKind)
     write64(Buf, getPPC64TocBase());
 }
 
 void PPC64::writePlt(uint8_t *Buf, uint64_t GotPltEntryAddr,
                      uint64_t PltEntryAddr, int32_t Index,
                      unsigned RelOff) const {
   uint64_t Off = GotPltEntryAddr - getPPC64TocBase();
 
+  if (Config->EKind == ELF64LEKind) {
+    // The most-common form of the plt stub. This assumes that the toc-pointer
+    // register is properly initalized, and that the stub must save the toc
+    // pointer value to the stack-save slot reserved for it (sp + 24).
+    // There are 2 other variants but we don't have to emit those until we add
+    // support for R_PPC64_REL24_NOTOC and R_PPC64_TOCSAVE relocations.
+    // We are missing a super simple optimization, where if the upper 16 bits of
+    // the offset are zero, then we can omit the addis instruction, and load
+    // r2 + lo-offset directly into r12. I decided to leave this out in the
+    // spirit of keeping it simple until we can link actual non-trivial
+    // programs.
+    write32(Buf +  0, 0xf8410018);                    // std     r2,24(r1)
+    write32(Buf +  4, 0x3d820000 | applyPPCHa(Off));  // addis   r12,r2, X@plt@to@ha
+    write32(Buf +  8, 0xe98c0000 | applyPPCLo(Off));  // ld      r12,X@plt@toc@l(r12)
+    write32(Buf + 12, 0x7d8903a6);                    // mtctr    r12
+    write32(Buf + 16, 0x4e800420);                    // bctr
+  } else {
     // FIXME: What we should do, in theory, is get the offset of the function
     // descriptor in the .opd section, and use that as the offset from %r2 (the
     // TOC-base pointer). Instead, we have the GOT-entry offset, and that will
     // be a pointer to the function descriptor in the .opd section. Using
     // this scheme is simpler, but requires an extra indirection per PLT dispatch.
-
     write32(Buf, 0xf8410028);                       // std %r2, 40(%r1)
     write32(Buf + 4, 0x3d620000 | applyPPCHa(Off)); // addis %r11, %r2, X@ha
     write32(Buf + 8, 0xe98b0000 | applyPPCLo(Off)); // ld %r12, X@l(%r11)
     write32(Buf + 12, 0xe96c0000);                  // ld %r11,0(%r12)
     write32(Buf + 16, 0x7d6903a6);                  // mtctr %r11
     write32(Buf + 20, 0xe84c0008);                  // ld %r2,8(%r12)
     write32(Buf + 24, 0xe96c0010);                  // ld %r11,16(%r12)
     write32(Buf + 28, 0x4e800420);                  // bctr
   }
+}
 
 static std::pair<RelType, uint64_t> toAddr16Rel(RelType Type, uint64_t Val) {
   uint64_t V = Val - PPC64TocOffset;
   switch (Type) {
   case R_PPC64_TOC16:
     return {R_PPC64_ADDR16, V};
   case R_PPC64_TOC16_DS:
     return {R_PPC64_ADDR16_DS, V};

lld/trunk/test/ELF/Inputs/shared-ppc64le.s

+    .text
+    .abiversion 2
+    .globl  foo
+    .p2align        4
+    .type   foo,@function
+
+foo:
+.Lfunc_begin0:
+  li 3, 55
+  blr
+  .long   0
+  .quad   0
+.Lfunc_end0:
+  .size foo, .Lfunc_end0-.Lfunc_begin0

lld/trunk/test/ELF/ppc64le-plt-stub.s

+// REQUIRES: ppc
+// RUN: llvm-mc -filetype=obj -triple=powerpc64le-unknown-linux %s -o %t.o
+// RUN: llvm-mc -filetype=obj -triple=powerpc64le-unknown-linux %p/Inputs/shared-ppc64le.s -o %t2.o
+// RUN: ld.lld -shared %t2.o -o %t2.so
+// RUN: ld.lld %t.o %t2.so -o %t
+// RUN: llvm-objdump -d %t | FileCheck %s
+
+// CHECK:          Disassembly of section .text:
+// CHECK:            _start:
+// CHECK:            bl .+24
+        .text
+        .abiversion 2
+        .globl  _start
+        .p2align        4
+        .type   _start,@function
+_start:
+.Lfunc_begin0:
+.Lfunc_gep0:
+  addis 2, 12, .TOC.-.Lfunc_gep0@ha
+  addi 2, 2, .TOC.-.Lfunc_gep0@l
+.Lfunc_lep0:
+  .localentry     _start, .Lfunc_lep0-.Lfunc_gep0
+  bl foo
+  nop
+  li 0, 1
+  sc
+  .size _start, .-.Lfunc_begin0
+
+
+
+// CHECK:          Disassembly of section .plt:
+// CHECK:          .plt:
+// CHECK-NEXT:            18 00 41 f8     std 2, 24(1)
+// CHECK-NEXT:            fe ff 82 3d     addis 12, 2, -2
+// CHECK-NEXT:            48 7f 8c e9     ld 12, 32584(12)
+// CHECK-NEXT:            a6 03 89 7d     mtctr 12
+// CHECK:                 20 04 80 4e     bctr