P0035R4: add predefined __STDCPP_DEFAULT_NEW_ALIGNMENT__ macro. By default, we

assume that ::operator new provides no more alignment than is necessary for any
primitive type, except when we're on a GNU OS, where glibc's malloc guarantees
to provide 64-bit alignment on 32-bit systems and 128-bit alignment on 64-bit
systems. This can be controlled by the command-line -fnew-alignment flag.

llvm-svn: 282974

1 files changed, 10 insertions, 0 deletions

diff --git a/clang/lib/Basic/TargetInfo.cpp b/clang/lib/Basic/TargetInfo.cpp
index d455fa3956d..19cab14cbf4 100644
--- a/clang/lib/Basic/TargetInfo.cpp
+++ b/clang/lib/Basic/TargetInfo.cpp
@@ -39,6 +39,13 @@ TargetInfo::TargetInfo(const llvm::Triple &T) : TargetOpts(), Triple(T) {
   SuitableAlign = 64;
   DefaultAlignForAttributeAligned = 128;
   MinGlobalAlign = 0;
+  // From the glibc documentation, on GNU systems, malloc guarantees 16-byte
+  // alignment on 64-bit systems and 8-byte alignment on 32-bit systems. See
+  // https://www.gnu.org/software/libc/manual/html_node/Malloc-Examples.html
+  if (T.isGNUEnvironment())
+    NewAlign = Triple.isArch64Bit() ? 128 : Triple.isArch32Bit() ? 64 : 0;
+  else
+    NewAlign = 0; // Infer from basic type alignment.
   HalfWidth = 16;
   HalfAlign = 16;
   FloatWidth = 32;
@@ -320,6 +327,9 @@ void TargetInfo::adjust(const LangOptions &Opts) {
     FloatFormat = &llvm::APFloat::IEEEsingle;
     LongDoubleFormat = &llvm::APFloat::IEEEquad;
   }
+
+  if (Opts.NewAlignOverride)
+    NewAlign = Opts.NewAlignOverride * getCharWidth();
 }
 
 bool TargetInfo::initFeatureMap(