Pack relocations for Android when possible.

[danalbert]

Summary

First supported by API 28.

clang/lib/Driver/ToolChains/Linux.cpp

if (Triple.isAndroid() && Triple.isAndroidVersionLT(29)) {
     // https://github.com/android/ndk/issues/1196
     // The unwinder used by the crash handler on versions of Android prior to
     // API 29 did not correctly handle binaries built with rosegment, which is
     // enabled by default for LLD. Android only supports LLD, so it's not an
     // issue that this flag is not accepted by other linkers.
     ExtraOpts.push_back("--no-rosegment");
   }
 
+  if (Triple.isAndroid() && !Triple.isAndroidVersionLT(28)) {
+    // Android supports relr packing starting with API 28 and had its own flavor
+    // (--pack-dyn-relocs=android) starting in API 23. It's possible to use both
+    // with --pack-dyn-relocs=android+relr, but we need to gather some data on
+    // the impact of that form before we can know if it's a good default.
+    //
+    // On the other hand, relr should always be an improvement.
+    ExtraOpts.push_back("--use-android-relr-tags");
+    ExtraOpts.push_back("--pack-dyn-relocs=relr");
+  }
+
   // Android ARM/AArch64 use max-page-size=4096 to reduce VMA usage. Note, lld
   // from 11 onwards default max-page-size to 65536 for both ARM and AArch64.
   if ((Triple.isARM() || Triple.isAArch64()) && Triple.isAndroid()) {
     ExtraOpts.push_back("-z");
     ExtraOpts.push_back("max-page-size=4096");
   }
 
   if (GCCInstallation.getParentLibPath().find("opt/rh/devtoolset") !=

clang/test/Driver/linux-ld.c

 // CHECK-ANDROID-ROSEGMENT-28: "--no-rosegment"
 //
 // RUN: %clang %s -### -o %t.o 2>&1 \
 // RUN:     --target=armv7-linux-android29 \
 // RUN:   | FileCheck --check-prefix=CHECK-ANDROID-ROSEGMENT-29 %s
 // CHECK-ANDROID-ROSEGMENT-29: "{{.*}}ld{{(.exe)?}}"
 // CHECK-ANDROID-ROSEGMENT-29-NOT: "--no-rosegment"
 
+// Check that we pass --pack-dyn-relocs=relr for API 28+ and not before.
+// RUN: %clang %s -### -o %t.o 2>&1 \
+// RUN:     --target=armv7-linux-android27 \
+// RUN:   | FileCheck --check-prefix=CHECK-ANDROID-RELR-27 %s
+// CHECK-ANDROID-RELR-27: "{{.*}}ld{{(.exe)?}}"
+// CHECK-ANDROID-RELR-27-NOT: "--pack-dyn-relocs=relr"
+// CHECK-ANDROID-RELR-27-NOT: "--pack-dyn-relocs=android+relr"
+//
+// RUN: %clang %s -### -o %t.o 2>&1 \
+// RUN:     --target=armv7-linux-android28 \
+// RUN:   | FileCheck --check-prefix=CHECK-ANDROID-RELR-28 %s
+// CHECK-ANDROID-RELR-28: "{{.*}}ld{{(.exe)?}}"
+// CHECK-ANDROID-RELR-28: "--use-android-relr-tags"
+// CHECK-ANDROID-RELR-28: "--pack-dyn-relocs=relr"
+// CHECK-ANDROID-RELR-28-NOT: "--pack-dyn-relocs=android+relr"
+
 // RUN: %clang %s -### -o %t.o 2>&1 \
 // RUN:     --target=armv7-linux-android21 \
 // RUN:   | FileCheck --check-prefix=CHECK-ANDROID-NOEXECSTACK %s
 // CHECK-ANDROID-NOEXECSTACK: "{{.*}}ld{{(.exe)?}}"
 // CHECK-ANDROID-NOEXECSTACK: "-z" "noexecstack"
 // CHECK-ANDROID-NOEXECSTACK-NOT: "-z" "execstack"
 // CHECK-ANDROID-NOEXECSTACK-NOT: "-z,execstack"
 // CHECK-ANDROID-NOEXECSTACK-NOT: "-zexecstack"