[PATCH] D28141: [COFF] Use 32-bit jump table entries in .rdata for Win64

[Reid Kleckner via Phabricator via llvm-commits]

rnk created this revision.
rnk added reviewers: majnemer, compnerd.
rnk added a subscriber: llvm-commits.

We were already using 32-bit jump table entries, but this was a
consequence of the default PIC model on Win64, and not an intentional
design decision. This patch ensures that we always use 32-bit label
difference jump table entries on Win64 regardless of the PIC model. This
is a good idea because it saves executable size and object file size.

Moving the jump tables to .rdata cleans up the disassembled object code
and reduces the available ROP targets, but it requires adding one more
RIP-relative lea to the code.  COFF doesn't have relocations to express
the difference between two arbitrary symbols, so we can't use the jump
table label in the label difference like we do elsewhere.

Fixes PR31488


https://reviews.llvm.org/D28141

Files:
  include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
  lib/CodeGen/TargetLoweringObjectFileImpl.cpp
  lib/Target/X86/X86ISelLowering.cpp
  lib/Target/X86/X86ISelLowering.h
  test/CodeGen/X86/win64-jumptable.ll

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