[MemDep] Handle gep with zeros for invariant.group

Summary:
gep 0, 0 is equivalent to bitcast. LLVM canonicalizes it
to getelementptr because it make SROA can then handle it.

Simple case like

    void g(A &a) {
        z(a);
        if (glob)
            a.foo();
    }
    void testG() {
        A a;
        g(a);
    }

was not devirtualized with -fstrict-vtable-pointers because luck of
handling for gep 0 in Memory Dependence Analysis

Reviewers: dberlin, nlewycky, chandlerc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28126

llvm-svn: 290763

1 files changed, 2 insertions, 1 deletions

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index e8dc8319d9c..ecf37bab55d 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -4904,7 +4904,8 @@ The existence of the ``invariant.group`` metadata on the instruction tells
 the optimizer that every ``load`` and ``store`` to the same pointer operand 
 within the same invariant group can be assumed to load or store the same  
 value (but see the ``llvm.invariant.group.barrier`` intrinsic which affects 
-when two pointers are considered the same).
+when two pointers are considered the same). Pointers returned by bitcast or
+getelementptr with only zero indices are considered the same.
 
 Examples: