Index: clang/lib/Driver/ToolChains/Arch/RISCV.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Arch/RISCV.cpp
+++ clang/lib/Driver/ToolChains/Arch/RISCV.cpp
@@ -379,7 +379,8 @@
   if (const Arg *A = Args.getLastArg(options::OPT_mabi_EQ))
     return A->getValue();
 
-  // FIXME: currently defaults to the soft-float ABIs. Will need to be
-  // expanded to select ilp32f, ilp32d, lp64f, lp64d when appropriate.
-  return Triple.getArch() == llvm::Triple::riscv32 ? "ilp32" : "lp64";
+  // 64-bit RISC-V Linux defaults to lp64d.
+  return Triple.getArch() == llvm::Triple::riscv32
+             ? "ilp32"
+             : Triple.getOS() == llvm::Triple::Linux ? "lp64d" : "lp64";
 }
Index: clang/test/Driver/riscv64-toolchain.c
===================================================================
--- clang/test/Driver/riscv64-toolchain.c
+++ clang/test/Driver/riscv64-toolchain.c
@@ -68,7 +68,7 @@
 // CXX-RV64-BAREMETAL-NOSYSROOT-LP64: "{{.*}}/Inputs/basic_riscv64_tree/lib/gcc/riscv64-unknown-elf/8.0.1{{/|\\\\}}crtend.o"
 
 // RUN: %clang %s -### -no-canonical-prefixes -fuse-ld=ld \
-// RUN:   -target riscv64-unknown-linux-gnu \
+// RUN:   -target riscv64-unknown-linux-gnu -mabi=lp64 \
 // RUN:   --gcc-toolchain=%S/Inputs/multilib_riscv_linux_sdk \
 // RUN:   --sysroot=%S/Inputs/multilib_riscv_linux_sdk/sysroot 2>&1 \
 // RUN:   | FileCheck -check-prefix=C-RV64-LINUX-MULTI-LP64 %s
@@ -84,7 +84,7 @@
 // C-RV64-LINUX-MULTI-LP64: "-L{{.*}}/Inputs/multilib_riscv_linux_sdk/sysroot/usr/lib64/lp64"
 
 // RUN: %clang %s -### -no-canonical-prefixes -fuse-ld=ld \
-// RUN:   -target riscv64-unknown-linux-gnu -march=rv64imafd -mabi=lp64d \
+// RUN:   -target riscv64-unknown-linux-gnu -march=rv64imafd \
 // RUN:   --gcc-toolchain=%S/Inputs/multilib_riscv_linux_sdk \
 // RUN:   --sysroot=%S/Inputs/multilib_riscv_linux_sdk/sysroot 2>&1 \
 // RUN:   | FileCheck -check-prefix=C-RV64-LINUX-MULTI-LP64D %s
Index: clang/test/Preprocessor/riscv-target-features.c
===================================================================
--- clang/test/Preprocessor/riscv-target-features.c
+++ clang/test/Preprocessor/riscv-target-features.c
@@ -50,7 +50,7 @@
 
 // RUN: %clang -target riscv32-unknown-linux-gnu -march=rv32ifd -x c -E -dM %s \
 // RUN: -o - | FileCheck --check-prefix=CHECK-SOFT %s
-// RUN: %clang -target riscv64-unknown-linux-gnu -march=rv64ifd -x c -E -dM %s \
+// RUN: %clang -target riscv64-unknown-linux-gnu -march=rv64ifd -mabi=lp64 -x c -E -dM %s \
 // RUN: -o - | FileCheck --check-prefix=CHECK-SOFT %s
 // CHECK-SOFT: __riscv_float_abi_soft 1
 // CHECK-SOFT-NOT: __riscv_float_abi_single
@@ -66,7 +66,7 @@
 
 // RUN: %clang -target riscv32-unknown-linux-gnu -march=rv32ifd -mabi=ilp32d -x c -E -dM %s \
 // RUN: -o - | FileCheck --check-prefix=CHECK-DOUBLE %s
-// RUN: %clang -target riscv64-unknown-linux-gnu -march=rv64ifd -mabi=lp64d -x c -E -dM %s \
+// RUN: %clang -target riscv64-unknown-linux-gnu -march=rv64ifd -x c -E -dM %s \
 // RUN: -o - | FileCheck --check-prefix=CHECK-DOUBLE %s
 // CHECK-DOUBLE: __riscv_float_abi_double 1
 // CHECK-DOUBLE-NOT: __riscv_float_abi_soft