[PATCH] D80055: Diagnose union tail padding

[JF Bastien via Phabricator via cfe-commits]

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Comment at: clang/test/CodeGen/union-tail-padding.c:9
+// RUN: %clang_cc1 -Wunion-tail-padding -xc++ -std=c++17 -triple aarch64-apple-darwin %s -emit-llvm -o /dev/null -verify
+// RUN: %clang_cc1 -Wunion-tail-padding -xc++ -std=c++2a -triple aarch64-apple-darwin %s -emit-llvm -o /dev/null -verify
+
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This test, while it might be surprising, is testing the current behavior of clang. I haven't validated that it's correct for each standard.


>From a discussion in D68115 <https://reviews.llvm.org/D68115>, it seems like union tail padding is often surprising to folks, compiler implementors included. This new diagnostic, -Wunion-tail-padding, emits a diagnostic when a union's tail is left undefined when develoeprs might have reasonably expected that it would not be. I've included -Wunion-tail-padding under the umbrella of a new diagnostic group, -Wpadding, since it often seems like padding in general is a source of pain. In particular, padding can leak secret information to malicious attackers. Here we focus on the places where padding is *unitialized*, not where a structure or union has padding.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D80055

Files:
  clang/include/clang/Basic/DiagnosticFrontendKinds.td
  clang/include/clang/Basic/DiagnosticGroups.td
  clang/lib/CodeGen/CGExprConstant.cpp
  clang/lib/CodeGen/CodeGenModule.cpp
  clang/lib/CodeGen/ConstantEmitter.h
  clang/test/CodeGen/union-tail-padding.c

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