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, 5 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InlineCost.cpp b/llvm/lib/Analysis/InlineCost.cpp
index aa46b68..ee36c6e5 100644
--- a/llvm/lib/Analysis/InlineCost.cpp
+++ b/llvm/lib/Analysis/InlineCost.cpp
@@ -900,6 +900,11 @@ bool CallAnalyzer::visitCallSite(CallSite CS) {
       default:
         return Base::visitCallSite(CS);
 
+      case Intrinsic::load_relative:
+        // This is normally lowered to 4 LLVM instructions.
+        Cost += 3 * InlineConstants::InstrCost;
+        return false;
+
       case Intrinsic::memset:
       case Intrinsic::memcpy:
       case Intrinsic::memmove: