[Driver] Add -fandroid-pad-segment/-fno-android-pad-segment (#77244)

-fandroid-pad-segment is an Android-specific opt-in option that
links in crt_pad_segment.o (beside other crt*.o relocatable files).

crt_pad_segment.o contains a note section, which will be included in the
linker-created PT_NOTE segment. This PT_NOTE tell Bionic that: when
create a map for a PT_LOAD segment, extend the end to cover the gap so
that we will have fewer kernel 'struct vm_area_struct' objects when
page_size < MAXPAGESIZE.

See also https://sourceware.org/bugzilla/show_bug.cgi?id=31076

Link: https://r.android.com/2902180

Author: MaskRay

clang/include/clang/Driver/Options.td

@@ -6298,6 +6298,11 @@ def fno_sycl : Flag<["-"], "fno-sycl">,
   Visibility<[ClangOption, CLOption]>,
   Group<sycl_Group>, HelpText<"Disables SYCL kernels compilation for device">;
 
+// OS-specific options
+let Flags = [TargetSpecific] in {
+defm android_pad_segment : BooleanFFlag<"android-pad-segment">, Group<f_Group>;
+} // let Flags = [TargetSpecific]
+
 //===----------------------------------------------------------------------===//
 // FLangOption + NoXarchOption
 //===----------------------------------------------------------------------===//

clang/lib/Driver/ToolChains/Gnu.cpp

@@ -508,6 +508,11 @@ void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
 
     // Add crtfastmath.o if available and fast math is enabled.
     ToolChain.addFastMathRuntimeIfAvailable(Args, CmdArgs);
+
+    if (isAndroid && Args.hasFlag(options::OPT_fandroid_pad_segment,
+                                  options::OPT_fno_android_pad_segment, false))
+      CmdArgs.push_back(
+          Args.MakeArgString(ToolChain.GetFilePath("crt_pad_segment.o")));
   }
 
   Args.addAllArgs(CmdArgs, {options::OPT_L, options::OPT_u});

clang/test/Driver/Inputs/basic_android_tree/sysroot/usr/lib/crt_pad_segment.o

@@ -1347,7 +1347,24 @@
 // RUN:     --sysroot=%S/Inputs/basic_android_tree/sysroot \
 // RUN:   | FileCheck --check-prefix=CHECK-ANDROID-PTHREAD-LINK %s
 // CHECK-ANDROID-PTHREAD-LINK-NOT: argument unused during compilation: '-pthread'
-//
+
+/// Check -fandroid-pad-segment.
+// RUN: %clang -### %s --target=aarch64-linux-android -rtlib=platform --unwindlib=platform \
+// RUN:   --gcc-toolchain="" --sysroot=%S/Inputs/basic_android_tree/sysroot \
+// RUN:   -fandroid-pad-segment 2>&1 | FileCheck --check-prefix=CHECK-ANDROID-PAD-PHDR %s
+// CHECK-ANDROID-PAD-PHDR: "{{.*}}ld{{(.exe)?}}" "--sysroot=[[SYSROOT:[^"]+]]"
+// CHECK-ANDROID-PAD-PHDR: "[[SYSROOT]]/usr/lib/crtbegin_dynamic.o" "[[SYSROOT]]/usr/lib/crt_pad_segment.o"
+
+// RUN: %clang -### %s --target=aarch64-linux-android -rtlib=platform --unwindlib=platform \
+// RUN:   --gcc-toolchain="" --sysroot=%S/Inputs/basic_android_tree/sysroot \
+// RUN:   -fandroid-pad-segment -fno-android-pad-segment 2>&1 | FileCheck --check-prefix=CHECK-NO-ANDROID-PAD-PHDR %s
+// CHECK-NO-ANDROID-PAD-PHDR: "{{.*}}ld{{(.exe)?}}" "--sysroot=[[SYSROOT:[^"]+]]"
+// CHECK-NO-ANDROID-PAD-PHDR: "[[SYSROOT]]/usr/lib/crtbegin_dynamic.o"
+// CHECK-NO-ANDROID-PAD-PHDR-NOT: crt_pad_segment.o"
+
+// RUN: not %clang -### %s --target=aarch64-linux -fandroid-pad-segment 2>&1 | FileCheck --check-prefix=ERR-ANDROID-PAD-EHDR %s
+// ERR-ANDROID-PAD-EHDR: error: unsupported option '-fandroid-pad-segment' for target 'aarch64-linux'
+
 // Check linker invocation on a Debian LoongArch sysroot.
 // RUN: %clang -### %s -no-pie 2>&1 \
 // RUN:     --target=loongarch64-linux-gnu -rtlib=platform --unwindlib=platform \