[AIX][TLS] Add target attribute for -maix-small-local-exec-tls option.

This patch adds a target attribute for an AIX-specific option that
informs the compiler that it can use a faster access sequence for the
local-exec TLS model (formally named aix-small-local-exec-tls).

The Clang portion of this option is in D155544.
The initial implementation to generate the faster access sequence is in
D155600.

Differential Revision: https://reviews.llvm.org/D156203

Author: amy-kwan

llvm/lib/Target/PowerPC/PPC.td

@@ -318,6 +318,17 @@ def FeaturePrivileged :
   SubtargetFeature<"privileged", "HasPrivileged", "true",
                    "Add privileged instructions">;
 
+// Specifies that local-exec TLS accesses in any function with this target
+// attribute should use the optimized TOC-free sequence (where the offset is an
+// immediate off of R13 for which the linker might add fix-up code if the
+// immediate is too large).
+// Clearly, this isn't really a feature of the subtarget, but is used as a
+// convenient way to affect code generation for individual functions.
+def FeatureAIXLocalExecTLS :
+  SubtargetFeature<"aix-small-local-exec-tls", "HasAIXSmallLocalExecTLS", "true",
+                   "Produce a TOC-free local-exec TLS sequence for this function "
+                   "for 64-bit AIX">;
+
 def FeaturePredictableSelectIsExpensive :
   SubtargetFeature<"predictable-select-expensive",
                    "PredictableSelectIsExpensive",

llvm/lib/Target/PowerPC/PPCSubtarget.cpp

@@ -123,6 +123,11 @@ void PPCSubtarget::initSubtargetFeatures(StringRef CPU, StringRef TuneCPU,
 
   // Determine endianness.
   IsLittleEndian = TM.isLittleEndian();
+
+  if (HasAIXSmallLocalExecTLS && (!TargetTriple.isOSAIX() || !IsPPC64))
+    report_fatal_error(
+      "The aix-small-local-exec-tls attribute is only supported on AIX in "
+      "64-bit mode.\n", false);
 }
 
 bool PPCSubtarget::enableMachineScheduler() const { return true; }

llvm/test/CodeGen/PowerPC/check-aix-small-local-exec-tls-opt-IRattribute.ll

@@ -0,0 +1,23 @@
+; RUN: llc -mtriple powerpc64-ibm-aix-xcoff -ppc-asm-full-reg-names \
+; RUN:   < %s | FileCheck %s
+; RUN: not llc -mtriple powerpc-ibm-aix-xcoff -ppc-asm-full-reg-names \
+; RUN:   < %s 2>&1 | FileCheck %s --check-prefix=CHECK-NOT-SUPPORTED
+; RUN: not llc -mtriple powerpc64le-unknown-linux-gnu -ppc-asm-full-reg-names \
+; RUN:   < %s 2>&1 | FileCheck %s --check-prefix=CHECK-NOT-SUPPORTED
+
+define dso_local signext i32 @testWithIRAttr() #0 {
+entry:
+  ret i32 0
+}
+; Check that the aix-small-local-exec-tls attribute is not supported on Linux and AIX (32-bit).
+; CHECK-NOT-SUPPORTED: The aix-small-local-exec-tls attribute is only supported on AIX in 64-bit mode.
+
+; Make sure that the test was actually compiled successfully after using the
+; aix-small-local-exec-tls attribute.
+; CHECK-LABEL: testWithIRAttr:
+; CHECK:        li r3, 0
+; CHECK-NEXT:   blr
+
+
+attributes #0 = { "target-features"="+aix-small-local-exec-tls" }
+

llvm/test/CodeGen/PowerPC/check-aix-small-local-exec-tls-opt.ll

@@ -0,0 +1,22 @@
+; RUN: llc -mtriple powerpc64-ibm-aix-xcoff -mattr=+aix-small-local-exec-tls \
+; RUN:   -ppc-asm-full-reg-names < %s | FileCheck %s
+; RUN: not llc -mtriple powerpc-ibm-aix-xcoff -mattr=+aix-small-local-exec-tls \
+; RUN:   -ppc-asm-full-reg-names < %s 2>&1 | \
+; RUN:   FileCheck %s --check-prefix=CHECK-NOT-SUPPORTED
+; RUN: not llc -mtriple powerpc64le-unknown-linux-gnu -mattr=+aix-small-local-exec-tls \
+; RUN:   -ppc-asm-full-reg-names < %s 2>&1 | \
+; RUN:   FileCheck %s --check-prefix=CHECK-NOT-SUPPORTED
+
+define dso_local signext i32 @testNoIRAttr() {
+entry:
+  ret i32 0
+}
+
+; Check that the aix-small-local-exec-tls attribute is not supported on Linux and AIX (32-bit).
+; CHECK-NOT-SUPPORTED: The aix-small-local-exec-tls attribute is only supported on AIX in 64-bit mode.
+
+; Make sure that the test was actually compiled successfully after using the
+; aix-small-local-exec-tls attribute.
+; CHECK-LABEL: testNoIRAttr:
+; CHECK:        li r3, 0
+; CHECK-NEXT:   blr