[DSE] Use precise loc for memcpy_chk during overwrite checks.

[fhahn]

Summary

Extends the logic from D115167 to memcpy_chk.

Depends on D115167

[asbirlea]

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

885

nit: will this be further extended to need the switch? or include in the if ( ... && (F == LibFunc_memset_chk || F == LibFunc_memcpy_chk))?

[fhahn]

Thanks! Update to use if and rebase, I am planning to land this soon.

[fhahn]

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

885

I think memset_chk and memcpy_chk are the main ones, so I updated the code to use an if instead of switch. We can use a switch if more cases are added, thanks!

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

DSEState(Function &F, AliasAnalysis &AA, MemorySSA &MSSA, DominatorTree &DT,
 
     CodeMetrics::collectEphemeralValues(&F, &AC, EphValues);
   }
 
   LocationSize strengthenLocationSize(const Instruction *I,
                                       LocationSize Size) const {
     if (auto *CB = dyn_cast<CallBase>(I)) {
       LibFunc F;
-      if (TLI.getLibFunc(*CB, F) && TLI.has(F) && F == LibFunc_memset_chk) {
+      if (TLI.getLibFunc(*CB, F) && TLI.has(F)) {
         // Use the precise location size specified by the 3rd argument during
         // for determining KillingI overwrites DeadLoc if it is a memset_chk
         // instruction. memset_chk will write either the amount specified as 3rd
         // argument or the function will immediately abort and exit the program.
         // NOTE: AA may determine NoAlias if it can prove that the access size
         // is larger than the allocation size due to that being UB. To avoid
         // returning potentially invalid NoAlias results by AA, limit the use of
         // the precise location size to isOverwrite.
+
+          switch (F) {
+          default:
+            break;
+          case LibFunc_memset_chk:
+          case LibFunc_memcpy_chk:
             if (const auto *Len = dyn_cast<ConstantInt>(CB->getArgOperand(2)))
               return LocationSize::precise(Len->getZExtValue());
+            break;
+          }
       }
     }
     return Size;
   }
 
   /// Return 'OW_Complete' if a store to the 'KillingLoc' location (by \p
   /// KillingI instruction) completely overwrites a store to the 'DeadLoc'
   /// location (by \p DeadI instruction).

llvm/test/Transforms/DeadStoreElimination/libcalls-chk.ll

 ;
   %call = tail call ptr @strncpy(ptr %out1, ptr %in, i64 100)
   %call.2 = tail call ptr @__memset_chk(ptr %out2, i32 42, i64 100, i64 %n)
   ret void
 }
 
 define void @dse_strncpy_memcpy_chk_test1(ptr noalias %out, ptr noalias %in, i64 %n) {
 ; CHECK-LABEL: @dse_strncpy_memcpy_chk_test1(
-; CHECK-NEXT:    store i32 0, ptr [[OUT:%.*]], align 4
-; CHECK-NEXT:    [[CALL_1:%.*]] = tail call ptr @__memcpy_chk(ptr [[OUT]], ptr [[IN:%.*]], i64 100, i64 [[N:%.*]])
+; CHECK-NEXT:    [[CALL_1:%.*]] = tail call ptr @__memcpy_chk(ptr [[OUT:%.*]], ptr [[IN:%.*]], i64 100, i64 [[N:%.*]])
 ; CHECK-NEXT:    ret void
 ;
   store i32 0, ptr %out
   %call.1 = tail call ptr @__memcpy_chk(ptr %out, ptr %in, i64 100, i64 %n)
   ret void
 }
 
 define void @dse_strncpy_memcpy_chk_test2(ptr noalias %out, ptr noalias %in, i64 %n) {