[CodeGenPrepare] Don't sink non-cheap addrspacecasts.

Summary:
Previously, CGP would unconditionally sink addrspacecast instructions,
even going so far as to sink them into a loop.

Now we check that the cast is "cheap", as defined by TLI.

We introduce a new "is-cheap" function to TLI rather than using
isNopAddrSpaceCast because some GPU platforms want the ability to ask
for non-nop casts to be sunk.

Reviewers: arsenm, tra

Subscribers: jholewinski, wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D26923

llvm-svn: 287591

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index 87ad3075873..68f090ccee2 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -927,6 +927,14 @@ static bool SinkCast(CastInst *CI) {
 ///
 static bool OptimizeNoopCopyExpression(CastInst *CI, const TargetLowering &TLI,
                                        const DataLayout &DL) {
+  // Sink only "cheap" (or nop) address-space casts.  This is a weaker condition
+  // than sinking only nop casts, but is helpful on some platforms.
+  if (auto *ASC = dyn_cast<AddrSpaceCastInst>(CI)) {
+    if (!TLI.isCheapAddrSpaceCast(ASC->getSrcAddressSpace(),
+                                  ASC->getDestAddressSpace()))
+      return false;
+  }
+
   // If this is a noop copy,
   EVT SrcVT = TLI.getValueType(DL, CI->getOperand(0)->getType());
   EVT DstVT = TLI.getValueType(DL, CI->getType());