Allow call-slop optzn for destinations with a suitable dereferenceable attribute

[dotdash]

Summary

Currently, call slot optimization requires that if the destination is an
argument, the argument has the sret attribute. This is to ensure that
the memory access won't trap. In addition to sret, we can also allow the
optimization to happen for arguments that have the new dereferenceable
attribute, which gives the same guarantee.

[hfinkel]

lib/Transforms/Scalar/MemCpyOptimizer.cpp

637

But if you do it this way you're now *requiring* the dereferenceable attribute, and we don't want to do that.

What you want to do is keep the existing logic for sret, and just add additional support for dereferenceable.

[dotdash]

lib/Transforms/Scalar/MemCpyOptimizer.cpp

637

How so? The new check just skips the sret check if we already know that we have enough deferenceable bytes.

If there is no dereferenceable attribute, getDereferenceableBytes() returns 0, which is < srcSize and so we do the sret check, and only if that fails, false is returned.

[hfinkel]

LGTM, thanks!

lib/Transforms/Scalar/MemCpyOptimizer.cpp

637

You're right; sorry about that.

Closed by commit rL219950 (authored by bsteinbr).

lib/Transforms/Scalar/MemCpyOptimizer.cpp

if (!destArraySize)
       return false;
 
     uint64_t destSize = DL->getTypeAllocSize(A->getAllocatedType()) *
       destArraySize->getZExtValue();
 
     if (destSize < srcSize)
       return false;
   } else if (Argument *A = dyn_cast<Argument>(cpyDest)) {
+    if (A->getDereferenceableBytes() < srcSize) {
       // If the destination is an sret parameter then only accesses that are
       // outside of the returned struct type can trap.
       if (!A->hasStructRetAttr())
         return false;
 
       Type *StructTy = cast<PointerType>(A->getType())->getElementType();
       if (!StructTy->isSized()) {
         // The call may never return and hence the copy-instruction may never
         // be executed, and therefore it's not safe to say "the destination
         // has at least <cpyLen> bytes, as implied by the copy-instruction",
         return false;
       }
 
       uint64_t destSize = DL->getTypeAllocSize(StructTy);
       if (destSize < srcSize)
         return false;
+    }
   } else {
     return false;
   }
 
   // Check that dest points to memory that is at least as aligned as src.
   unsigned srcAlign = srcAlloca->getAlignment();
   if (!srcAlign)
     srcAlign = DL->getABITypeAlignment(srcAlloca->getAllocatedType());

test/Transforms/MemCpyOpt/callslot_deref.ll

+; RUN: opt < %s -S -basicaa -memcpyopt | FileCheck %s
+target datalayout = "e-i64:64-f80:128-n8:16:32:64-S128"
+
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture readonly, i64, i32, i1) unnamed_addr nounwind
+declare void @llvm.memset.p0i8.i64(i8* nocapture, i8, i64, i32, i1) nounwind
+
+; all bytes of %dst that are touch by the memset are dereferenceable
+define void @must_remove_memcpy(i8* noalias nocapture dereferenceable(4096) %dst) {
+; CHECK-LABEL: @must_remove_memcpy(
+; CHECK: call void @llvm.memset.p0i8.i64
+; CHECK-NOT: call void @llvm.memcpy.p0i8.p0i8.i64
+  %src = alloca [4096 x i8], align 1
+  %p = getelementptr inbounds [4096 x i8]* %src, i64 0, i64 0
+  call void @llvm.memset.p0i8.i64(i8* %p, i8 0, i64 4096, i32 1, i1 false)
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %dst, i8* %p, i64 4096, i32 1, i1 false) #2
+  ret void
+}
+
+; memset touch more bytes than those guaranteed to be dereferenceable
+define void @must_not_remove_memcpy(i8* noalias nocapture dereferenceable(1024) %dst) {
+; CHECK-LABEL: @must_not_remove_memcpy(
+; CHECK: call void @llvm.memset.p0i8.i64
+; CHECK: call void @llvm.memcpy.p0i8.p0i8.i64
+  %src = alloca [4096 x i8], align 1
+  %p = getelementptr inbounds [4096 x i8]* %src, i64 0, i64 0
+  call void @llvm.memset.p0i8.i64(i8* %p, i8 0, i64 4096, i32 1, i1 false)
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %dst, i8* %p, i64 4096, i32 1, i1 false) #2
+  ret void
+}