[AIX][TLS][clang] Add -maix-small-local-exec-tls clang option.

This patch adds the clang portion of an AIX-specific option to inform
the compiler that it can use a faster access sequence for the local-exec
TLS model (formally named aix-small-local-exec-tls).

This patch only adds the frontend portion of the option, building upon:

Backend portion of the option (D156203)
Backend patch that utilizes this option to actually produce the faster access sequence (D155600)

Differential Revision: https://reviews.llvm.org/D155544

Author: amy-kwan

clang/docs/ReleaseNotes.rst

@@ -336,6 +336,12 @@ CUDA Support
 AIX Support
 ^^^^^^^^^^^
 
+- Introduced the ``-maix-small-local-exec-tls`` option to produce a faster
+  access sequence for local-exec TLS variables where the offset from the TLS
+  base is encoded as an immediate operand.
+  This access sequence is not used for TLS variables larger than 32KB, and is
+  currently only supported on 64-bit mode.
+
 WebAssembly Support
 ^^^^^^^^^^^^^^^^^^^
 

clang/include/clang/Driver/Options.td

@@ -4723,6 +4723,12 @@ def mrop_protect : Flag<["-"], "mrop-protect">,
 def mprivileged : Flag<["-"], "mprivileged">,
     Group<m_ppc_Features_Group>;
 } // let Flags = [TargetSpecific]
+def maix_small_local_exec_tls : Flag<["-"], "maix-small-local-exec-tls">,
+  Group<m_ppc_Features_Group>,
+  HelpText<"Produce a faster access sequence for local-exec TLS variables "
+           "where the offset from the TLS base is encoded as an "
+           "immediate operand (AIX 64-bit only). "
+           "This access sequence is not used for variables larger than 32KB.">;
 def maix_struct_return : Flag<["-"], "maix-struct-return">,
   Group<m_Group>, Visibility<[ClangOption, CC1Option]>,
   HelpText<"Return all structs in memory (PPC32 only)">,

clang/lib/Basic/Targets/PPC.cpp

@@ -77,6 +77,8 @@ bool PPCTargetInfo::handleTargetFeatures(std::vector<std::string> &Features,
       HasROPProtect = true;
     } else if (Feature == "+privileged") {
       HasPrivileged = true;
+    } else if (Feature == "+aix-small-local-exec-tls") {
+      HasAIXSmallLocalExecTLS = true;
     } else if (Feature == "+isa-v206-instructions") {
       IsISA2_06 = true;
     } else if (Feature == "+isa-v207-instructions") {
@@ -541,6 +543,10 @@ bool PPCTargetInfo::initFeatureMap(
   // Privileged instructions are off by default.
   Features["privileged"] = false;
 
+  // The code generated by the -maix-small-local-exec-tls option is turned
+  // off by default.
+  Features["aix-small-local-exec-tls"] = false;
+
   Features["spe"] = llvm::StringSwitch<bool>(CPU)
                         .Case("8548", true)
                         .Case("e500", true)
@@ -635,6 +641,14 @@ bool PPCTargetInfo::initFeatureMap(
     return false;
   }
 
+  if (llvm::is_contained(FeaturesVec, "+aix-small-local-exec-tls")) {
+    if (!getTriple().isOSAIX() || !getTriple().isArch64Bit()) {
+      Diags.Report(diag::err_opt_not_valid_on_target)
+         << "-maix-small-local-exec-tls";
+      return false;
+    }
+  }
+
   return TargetInfo::initFeatureMap(Features, Diags, CPU, FeaturesVec);
 }
 
@@ -676,6 +690,7 @@ bool PPCTargetInfo::hasFeature(StringRef Feature) const {
       .Case("mma", HasMMA)
       .Case("rop-protect", HasROPProtect)
       .Case("privileged", HasPrivileged)
+      .Case("aix-small-local-exec-tls", HasAIXSmallLocalExecTLS)
       .Case("isa-v206-instructions", IsISA2_06)
       .Case("isa-v207-instructions", IsISA2_07)
       .Case("isa-v30-instructions", IsISA3_0)

clang/lib/Basic/Targets/PPC.h

@@ -60,6 +60,7 @@ class LLVM_LIBRARY_VISIBILITY PPCTargetInfo : public TargetInfo {
   bool HasMMA = false;
   bool HasROPProtect = false;
   bool HasPrivileged = false;
+  bool HasAIXSmallLocalExecTLS = false;
   bool HasVSX = false;
   bool UseCRBits = false;
   bool HasP8Vector = false;

clang/test/Driver/aix-small-local-exec-tls.c

@@ -0,0 +1,30 @@
+// RUN: %clang -target powerpc64-unknown-aix -S -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang -target powerpc-unknown-aix -S -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang -target powerpc64le-unknown-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang -target powerpc64-unknown-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
+
+// RUN: %clang -target powerpc64-unknown-aix -maix-small-local-exec-tls -S -emit-llvm \
+// RUN:    %s -o - | FileCheck %s --check-prefix=CHECK-AIX_SMALL_LOCALEXEC_TLS
+
+// RUN: not %clang -target powerpc-unknown-aix -maix-small-local-exec-tls \
+// RUN:    -fsyntax-only %s 2>&1 | FileCheck --check-prefix=CHECK-UNSUPPORTED-AIX32 %s
+// RUN: not %clang -target powerpc64le-unknown-linux-gnu -maix-small-local-exec-tls \
+// RUN:    -fsyntax-only %s 2>&1 | FileCheck --check-prefix=CHECK-UNSUPPORTED-LINUX %s
+// RUN: not %clang -target powerpc64-unknown-linux-gnu -maix-small-local-exec-tls \
+// RUN:    -fsyntax-only %s 2>&1 | FileCheck --check-prefix=CHECK-UNSUPPORTED-LINUX %s
+
+int test(void) {
+  return 0;
+}
+
+// CHECK: test() #0 {
+// CHECK: attributes #0 = {
+// CHECK-SAME: -aix-small-local-exec-tls
+
+// CHECK-UNSUPPORTED-AIX32: option '-maix-small-local-exec-tls' cannot be specified on this target
+// CHECK-UNSUPPORTED-LINUX: option '-maix-small-local-exec-tls' cannot be specified on this target
+
+// CHECK-AIX_SMALL_LOCALEXEC_TLS: test() #0 {
+// CHECK-AIX_SMALL_LOCALEXEC_TLS: attributes #0 = {
+// CHECK-AIX_SMALL_LOCALEXEC_TLS-SAME: +aix-small-local-exec-tls
+