Introduce llvm.load.relative intrinsic.

This intrinsic takes two arguments, ``%ptr`` and ``%offset``. It loads
a 32-bit value from the address ``%ptr + %offset``, adds ``%ptr`` to that
value and returns it. The constant folder specifically recognizes the form of
this intrinsic and the constant initializers it may load from; if a loaded
constant initializer is known to have the form ``i32 trunc(x - %ptr)``,
the intrinsic call is folded to ``x``.

LLVM provides that the calculation of such a constant initializer will
not overflow at link time under the medium code model if ``x`` is an
``unnamed_addr`` function. However, it does not provide this guarantee for
a constant initializer folded into a function body. This intrinsic can be
used to avoid the possibility of overflows when loading from such a constant.

Differential Revision: http://reviews.llvm.org/D18367

llvm-svn: 267223

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/tools/opt/opt.cpp b/llvm/tools/opt/opt.cpp
index 641735c2998..91993bfaa32 100644
--- a/llvm/tools/opt/opt.cpp
+++ b/llvm/tools/opt/opt.cpp
@@ -353,6 +353,7 @@ int main(int argc, char **argv) {
   initializeDwarfEHPreparePass(Registry);
   initializeSafeStackPass(Registry);
   initializeSjLjEHPreparePass(Registry);
+  initializePreISelIntrinsicLoweringPass(Registry);
 
 #ifdef LINK_POLLY_INTO_TOOLS
   polly::initializePollyPasses(Registry);