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

3 files changed, 9 insertions, 0 deletions

diff --git a/llvm/include/llvm/CodeGen/Passes.h b/llvm/include/llvm/CodeGen/Passes.h
index 30630dcf644..589c0500a41 100644
--- a/llvm/include/llvm/CodeGen/Passes.h
+++ b/llvm/include/llvm/CodeGen/Passes.h
@@ -680,6 +680,11 @@ namespace llvm {
   ///
   ModulePass *createLowerEmuTLSPass(const TargetMachine *TM);
 
+  /// This pass lowers the @llvm.load.relative intrinsic to instructions.
+  /// This is unsafe to do earlier because a pass may combine the constant
+  /// initializer into the load, which may result in an overflowing evaluation.
+  ModulePass *createPreISelIntrinsicLoweringPass();
+
   /// GlobalMerge - This pass merges internal (by default) globals into structs
   /// to enable reuse of a base pointer by indexed addressing modes.
   /// It can also be configured to focus on size optimizations only.
diff --git a/llvm/include/llvm/IR/Intrinsics.td b/llvm/include/llvm/IR/Intrinsics.td
index e49872043fd..f245df4ff91 100644
--- a/llvm/include/llvm/IR/Intrinsics.td
+++ b/llvm/include/llvm/IR/Intrinsics.td
@@ -669,6 +669,9 @@ def int_masked_scatter: Intrinsic<[],
 def int_bitset_test : Intrinsic<[llvm_i1_ty], [llvm_ptr_ty, llvm_metadata_ty],
                                 [IntrNoMem]>;
 
+def int_load_relative: Intrinsic<[llvm_ptr_ty], [llvm_ptr_ty, llvm_anyint_ty],
+                                 [IntrReadMem, IntrArgMemOnly]>;
+
 //===----------------------------------------------------------------------===//
 // Target-specific intrinsics
 //===----------------------------------------------------------------------===//
diff --git a/llvm/include/llvm/InitializePasses.h b/llvm/include/llvm/InitializePasses.h
index 638071fb11d..557f927a656 100644
--- a/llvm/include/llvm/InitializePasses.h
+++ b/llvm/include/llvm/InitializePasses.h
@@ -246,6 +246,7 @@ void initializePostOrderFunctionAttrsLegacyPassPass(PassRegistry&);
 void initializePostRAHazardRecognizerPass(PassRegistry&);
 void initializePostRASchedulerPass(PassRegistry&);
 void initializePostMachineSchedulerPass(PassRegistry&);
+void initializePreISelIntrinsicLoweringPass(PassRegistry&);
 void initializePrintFunctionPassWrapperPass(PassRegistry&);
 void initializePrintModulePassWrapperPass(PassRegistry&);
 void initializePrintBasicBlockPassPass(PassRegistry&);