[MemCpyOpt] Fix metadata merging during call slot optimization

[nikic]

Summary

Call slot optimization currently merges the metadata between the call and the load. However, we also need to merge in the metadata of the store.

Part of the reason why we might have gotten away with this previously is that usually the load and the store are the same instruction (a memcpy), this can only happen if call slot optimization occurs on an actual load/store pair.

This addresses the issue reported in https://reviews.llvm.org/D115615#3251386.

[jeroen.dobbelaere]

LGTM

[reames]

LGTM as well.

[hans]

lgtm

llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp

bool MemCpyOptPass::performCallSlotOptzn(Instruction *cpyLoad,
   // Update AA metadata
   // FIXME: MD_tbaa_struct and MD_mem_parallel_loop_access should also be
   // handled here, but combineMetadata doesn't support them yet
   unsigned KnownIDs[] = {LLVMContext::MD_tbaa, LLVMContext::MD_alias_scope,
                          LLVMContext::MD_noalias,
                          LLVMContext::MD_invariant_group,
                          LLVMContext::MD_access_group};
   combineMetadata(C, cpyLoad, KnownIDs, true);
+  if (cpyLoad != cpyStore)
+    combineMetadata(C, cpyStore, KnownIDs, true);
 
   ++NumCallSlot;
   return true;
 }
 
 /// We've found that the (upward scanning) memory dependence of memcpy 'M' is
 /// the memcpy 'MDep'. Try to simplify M to copy from MDep's input if we can.
 bool MemCpyOptPass::processMemCpyMemCpyDependence(MemCpyInst *M,

llvm/test/Transforms/MemCpyOpt/callslot_noalias.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -S -memcpyopt < %s | FileCheck %s
 
 declare void @func(i8* %dst)
 
-; TODO: The noalias metadata on the call is currently incorrect.
+; The noalias metadata from the call, the load and the store should be merged,
+; so that no metadata is left on the call.
 define i8 @test(i8* dereferenceable(1) noalias %dst) {
 ; CHECK-LABEL: @test(
 ; CHECK-NEXT:    [[TMP:%.*]] = alloca i8, align 1
-; CHECK-NEXT:    call void @func(i8* nocapture [[DST:%.*]]) #[[ATTR0:[0-9]+]], !noalias !0
+; CHECK-NEXT:    call void @func(i8* nocapture [[DST:%.*]]) #[[ATTR0:[0-9]+]]{{$}}
 ; CHECK-NEXT:    [[V2:%.*]] = load i8, i8* [[DST]], align 1, !alias.scope !0
 ; CHECK-NEXT:    ret i8 [[V2]]
 ;
   %tmp = alloca i8
   call void @func(i8* nocapture %tmp) nounwind, !noalias !0
   %v = load i8, i8* %tmp, !noalias !0
   store i8 %v, i8* %dst, !alias.scope !0
   %v2 = load i8, i8* %dst, !alias.scope !0
   ret i8 %v2
 }
 
 !0 = !{!1}
 !1 = !{!1, !2}
 !2 = !{!2}