[CHERI] No need to preserve tags when copying from NULL constants

arichardson · arichardson · commit f0277de5afc5 · 2022-10-06T21:56:26.000Z
Looking at the commit history the only test that triggers this
is specific to the MS C++ ABI, so I have not been able to create
a testcase.
diff --git a/clang/lib/CodeGen/CGExprCXX.cpp b/clang/lib/CodeGen/CGExprCXX.cpp
@@ -564,7 +564,8 @@ static void EmitNullBaseClassInitialization(CodeGenFunction &CGF,
       CGF.Builder.CreateMemCpy(
           CGF.Builder.CreateConstInBoundsByteGEP(DestPtr, StoreOffset),
           CGF.Builder.CreateConstInBoundsByteGEP(SrcPtr, StoreOffset),
-          StoreSizeVal);
+          StoreSizeVal, /* NULL constants never contain tags */
+          llvm::PreserveCheriTags::Unnecessary);
     }
 
   // Otherwise, just memset the whole thing to zero.  This is legal

Original file line number	Diff line number	Diff line change
`@@ -564,7 +564,8 @@ static void EmitNullBaseClassInitialization(CodeGenFunction &CGF,`
`564`	`564`	`CGF.Builder.CreateMemCpy(`
`565`	`565`	`CGF.Builder.CreateConstInBoundsByteGEP(DestPtr, StoreOffset),`
`566`	`566`	`CGF.Builder.CreateConstInBoundsByteGEP(SrcPtr, StoreOffset),`
`567`		`- StoreSizeVal);`
	`567`	`+ StoreSizeVal, /* NULL constants never contain tags */`
	`568`	`+ llvm::PreserveCheriTags::Unnecessary);`
`568`	`569`	`}`
`569`	`570`
`570`	`571`	`// Otherwise, just memset the whole thing to zero. This is legal`