[PATCH] D26566: Use PIC relocation model as default for PowerPC64 ELF

[Joerg Sonnenberger via llvm-commits]

joerg created this revision.
joerg added a reviewer: hfinkel.
joerg added a subscriber: llvm-commits.
joerg set the repository for this revision to rL LLVM.
Herald added a subscriber: nemanjai.

Most of the PowerPC64 code generation for the ELF ABI is already PIC. There are four main exceptions:
(1) Constant pointer arrays etc. should in writeable sections.
(2) The TOC restoration NOP after a call is needed for all global symbols. While GNU ld has a workaround
for questionable GCC self-calls, we trigger the checks for calls from COMDAT sections as they cross input
sections and are therefore not considered self-calls. The current decision is questionable and suboptimal,
but outside the scope of the change.
(3) TLS access can not use the initial-exec model.
(4) Jump tables should use relative addresses. Note that the current encoding doesn't work for the large
code model, but it is more compact than the default for any non-trivial jump table. Improving this is again
beyond the scope of this change.

At least (1) and (3) are assumptions made in target-independent code and introducing additional hooks is a bit messy.
Testing with clang shows that a -fPIC binary is 600KB smaller than the corresponding -fno-pic build. Separate testing
from improved jump table encodings would explain only about 100KB or so. The rest is expected to be a result of
more aggressive immediate forming for -fno-pic, where the -fPIC binary just uses TOC entries.

This change brings the LLVM output in line with the GCC output, other PPC64 compilers like XLC on AIX are known
to produce PIC by default as well. The relocation model can still be provided explicitly, i.e. when using MCJIT.

One test case for case (1) is included, other test cases with relocation mode sensitive behavior are wired to static for now.
They will be reviewed and adjusted separately.


Repository:
  rL LLVM

https://reviews.llvm.org/D26566

Files:
  lib/Target/PowerPC/PPCTargetMachine.cpp
  test/CodeGen/PowerPC/alias.ll
  test/CodeGen/PowerPC/cxx_tlscc64.ll
  test/CodeGen/PowerPC/fast-isel-br-const.ll
  test/CodeGen/PowerPC/fast-isel-load-store.ll
  test/CodeGen/PowerPC/func-addr-consts.ll
  test/CodeGen/PowerPC/func-addr.ll
  test/CodeGen/PowerPC/mcm-11.ll
  test/CodeGen/PowerPC/mcm-3.ll
  test/CodeGen/PowerPC/mcm-5.ll
  test/CodeGen/PowerPC/mcm-obj-2.ll
  test/CodeGen/PowerPC/mcm-obj.ll
  test/CodeGen/PowerPC/peephole-align.ll
  test/CodeGen/PowerPC/ppc64-calls.ll
  test/CodeGen/PowerPC/ppc64-nonfunc-calls.ll
  test/CodeGen/PowerPC/ppc64-sibcall-shrinkwrap.ll
  test/CodeGen/PowerPC/ppc64-sibcall.ll
  test/CodeGen/PowerPC/tailcall-string-rvo.ll
  test/CodeGen/PowerPC/tailcall1-64.ll
  test/CodeGen/PowerPC/tls.ll
  test/CodeGen/PowerPC/vsx.ll

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D26566.77671.patch
Type: text/x-patch
Size: 17469 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20161111/3e68193c/attachment.bin>