[lld][RISCV] Implement GP relaxation for R_RISCV_HI20/R_RISCV_LO12_I/R_RISCV_LO12_S.

This implements support for relaxing these relocations to use the GP
register to compute addresses of globals in the .sdata and .sbss
sections.

This feature is off by default and must be enabled by passing
--relax-gp to the linker.

The GP register might not always be the "global pointer". It can
be used for other purposes. See discussion here
https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/371

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D143673

2 files changed, 94 insertions, 0 deletions

diff --git a/lld/test/ELF/riscv-relax-hi20-lo12-pie.s b/lld/test/ELF/riscv-relax-hi20-lo12-pie.s
new file mode 100644
index 000000000000..87e4f8f000e5
--- /dev/null
+++ b/lld/test/ELF/riscv-relax-hi20-lo12-pie.s
@@ -0,0 +1,33 @@
+# REQUIRES: riscv
+# RUN: rm -rf %t && split-file %s %t && cd %t
+
+# RUN: llvm-mc -filetype=obj -triple=riscv32-unknown-elf -mattr=+relax a.s -o rv32.o
+# RUN: llvm-mc -filetype=obj -triple=riscv64-unknown-elf -mattr=+relax a.s -o rv64.o
+# RUN: llvm-mc -filetype=obj -triple=riscv64-unknown-elf -mattr=+relax a.s -o rv64-pie.o
+
+# RUN: ld.lld --relax-gp --undefined=__global_pointer$ rv32.o lds -pie -o rv32
+# RUN: ld.lld --relax-gp --undefined=__global_pointer$ rv64.o lds -shared -o rv64
+# RUN: llvm-objdump -td -M no-aliases --no-show-raw-insn rv32 | FileCheck %s
+# RUN: llvm-objdump -td -M no-aliases --no-show-raw-insn rv64 | FileCheck %s
+
+# CHECK:      lui     a0, 512
+# CHECK-NEXT: addi    a0, a0, 1
+# CHECK-NEXT: lw      a0, 1(a0)
+# CHECK-NEXT: sw      a0, 1(a0)
+
+#--- a.s
+.globl abs
+abs = 0x200001
+
+.global _start
+_start:
+  lui a0, %hi(abs)
+  addi a0, a0, %lo(abs)
+  lw a0, %lo(abs)(a0)
+  sw a0, %lo(abs)(a0)
+
+#--- lds
+SECTIONS {
+  .text : {*(.text) }
+  .sdata 0x200000 : {}
+}
diff --git a/lld/test/ELF/riscv-relax-hi20-lo12.s b/lld/test/ELF/riscv-relax-hi20-lo12.s
new file mode 100644
index 000000000000..feef08767c19
--- /dev/null
+++ b/lld/test/ELF/riscv-relax-hi20-lo12.s
@@ -0,0 +1,61 @@
+# REQUIRES: riscv
+# RUN: rm -rf %t && split-file %s %t && cd %t
+
+# RUN: llvm-mc -filetype=obj -triple=riscv32-unknown-elf -mattr=+relax a.s -o rv32.o
+# RUN: llvm-mc -filetype=obj -triple=riscv64-unknown-elf -mattr=+relax a.s -o rv64.o
+
+# RUN: ld.lld --relax-gp --undefined=__global_pointer$ rv32.o lds -o rv32
+# RUN: ld.lld --relax-gp --undefined=__global_pointer$ rv64.o lds -o rv64
+# RUN: llvm-objdump -td -M no-aliases --no-show-raw-insn rv32 | FileCheck %s
+# RUN: llvm-objdump -td -M no-aliases --no-show-raw-insn rv64 | FileCheck %s
+
+# CHECK: 00000028 l       .text {{0*}}0 a
+
+# CHECK-NOT:  lui
+# CHECK:      addi    a0, gp, -2048
+# CHECK-NEXT: lw      a0, -2048(gp)
+# CHECK-NEXT: sw      a0, -2048(gp)
+# CHECK-NOT:  lui
+# CHECK-NEXT: addi    a0, gp, 2047
+# CHECK-NEXT: lb      a0, 2047(gp)
+# CHECK-NEXT: sb      a0, 2047(gp)
+# CHECK-NEXT: lui     a0, 513
+# CHECK-NEXT: addi    a0, a0, 0
+# CHECK-NEXT: lw      a0, 0(a0)
+# CHECK-NEXT: sw      a0, 0(a0)
+# CHECK-EMPTY:
+# CHECK-NEXT: <a>:
+# CHECK-NEXT: addi a0, a0, 1
+
+#--- a.s
+.global _start
+_start:
+  lui a0, %hi(foo)
+  addi a0, a0, %lo(foo)
+  lw a0, %lo(foo)(a0)
+  sw a0, %lo(foo)(a0)
+  lui a0, %hi(bar)
+  addi a0, a0, %lo(bar)
+  lb a0, %lo(bar)(a0)
+  sb a0, %lo(bar)(a0)
+  lui a0, %hi(norelax)
+  addi a0, a0, %lo(norelax)
+  lw a0, %lo(norelax)(a0)
+  sw a0, %lo(norelax)(a0)
+a:
+  addi a0, a0, 1
+
+.section .sdata,"aw"
+foo:
+  .word 0
+  .space 4091
+bar:
+  .byte 0
+norelax:
+  .word 0
+
+#--- lds
+SECTIONS {
+  .text : {*(.text) }
+  .sdata 0x200000 : { }
+}