[TLS]: Clamp the alignment of TLS global variables if required by the target

Adding a module flag 'MaxTLSAlign' describing the maximum alignment a global TLS
variable can have. Optimizers are prevented from increasing the alignment of such
variables beyond this threshold.

Reviewed By: probinson

Differential Revision: https://reviews.llvm.org/D140123

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/IR/Module.cpp b/llvm/lib/IR/Module.cpp
index 49fadc9..3df1e7b 100644
--- a/llvm/lib/IR/Module.cpp
+++ b/llvm/lib/IR/Module.cpp
@@ -746,6 +746,13 @@ unsigned Module::getOverrideStackAlignment() const {
   return 0;
 }
 
+unsigned Module::getMaxTLSAlignment() const {
+  Metadata *MD = getModuleFlag("MaxTLSAlign");
+  if (auto *CI = mdconst::dyn_extract_or_null<ConstantInt>(MD))
+    return CI->getZExtValue();
+  return 0;
+}
+
 void Module::setOverrideStackAlignment(unsigned Align) {
   addModuleFlag(ModFlagBehavior::Error, "override-stack-alignment", Align);
 }