[JITLink][RISCV] Only generate PLT entries for external symbols

R_RISCV_CALL has been deprecated. [1] Both GCC and LLVM seem to not
generate it anymore and always use R_RISCV_CALL_PLT (even for calls that
do not need a PLT entry). Generating PLT entries based on relocation
type is not recommended and a better heuristic is to only generate them
when the target symbol is preemptable [2]. This patch implements this by
only generating PLT entries for undefined symbols.

[1] riscv-non-isa/riscv-elf-psabi-doc#340
[2] riscv-non-isa/riscv-elf-psabi-doc#98

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D149525

Author: mtvec

llvm/lib/ExecutionEngine/JITLink/ELF_riscv.cpp

@@ -76,7 +76,8 @@ class PerGraphGOTAndPLTStubsBuilder_ELF_riscv
   }
 
   bool isExternalBranchEdge(Edge &E) const {
-    return E.getKind() == R_RISCV_CALL_PLT;
+    return (E.getKind() == R_RISCV_CALL || E.getKind() == R_RISCV_CALL_PLT) &&
+           !E.getTarget().isDefined();
   }
 
 private:

llvm/test/ExecutionEngine/JITLink/RISCV/ELF_pc_indirect.s

@@ -5,11 +5,11 @@
 # RUN:     -o %t/elf_riscv32_sm_pic_reloc.o %s
 # RUN: llvm-jitlink -noexec \
 # RUN:     -slab-allocate 100Kb -slab-address 0x1ff00000 -slab-page-size 4096 \
-# RUN:     -abs external_func=0x1 -abs external_data=0x2 \
+# RUN:     -abs external_data=0x2 \
 # RUN:     -check %s %t/elf_riscv64_sm_pic_reloc.o
 # RUN: llvm-jitlink -noexec \
 # RUN:     -slab-allocate 100Kb -slab-address 0x1ff00000 -slab-page-size 4096 \
-# RUN:     -abs external_func=0x1 -abs external_data=0x2 \
+# RUN:     -abs external_data=0x2 \
 # RUN:     -check %s %t/elf_riscv32_sm_pic_reloc.o
 #
 # Test ELF small/PIC relocations
@@ -51,14 +51,34 @@ test_pcrel32_s:
         .size   test_pcrel32_s, .-test_pcrel32_s
 
 # Test R_RISCV_CALL
-# jitlink-check: decode_operand(test_call, 1) = ((external_func - test_call) + 0x800)[31:12]
-# jitlink-check: decode_operand(test_call+4, 2)[11:0] = (external_func - test_call)[11:0]
-        .globl test_call, external_func
+# jitlink-check: decode_operand(test_call, 1) = ((internal_func - test_call) + 0x800)[31:12]
+# jitlink-check: decode_operand(test_call+4, 2)[11:0] = (internal_func - test_call)[11:0]
+        .globl test_call
         .p2align  1
         .type  test_call,@function
 test_call:
-        .reloc ., R_RISCV_CALL, external_func
+        .reloc ., R_RISCV_CALL, internal_func
         auipc ra, 0
         jalr ra
         ret
         .size test_call, .-test_call
+
+# Test R_RISCV_CALL_PLT
+# jitlink-check: decode_operand(test_call_plt, 1) = ((internal_func - test_call_plt) + 0x800)[31:12]
+# jitlink-check: decode_operand(test_call_plt+4, 2)[11:0] = (internal_func - test_call_plt)[11:0]
+        .globl test_call_plt
+        .p2align  1
+        .type  test_call_plt,@function
+test_call_plt:
+        .reloc ., R_RISCV_CALL_PLT, internal_func
+        auipc ra, 0
+        jalr ra
+        ret
+        .size test_call_plt, .-test_call_plt
+
+       .globl   internal_func
+       .p2align  1
+       .type    internal_func,@function
+internal_func:
+       ret
+       .size internal_func, .-internal_func