[RISCV] Fix evaluation of %pcrel_lo

The following testcase

  function:
  .Lpcrel_label1:
  	auipc	a0, %pcrel_hi(other_function)
  	addi	a1, a0, %pcrel_lo(.Lpcrel_label1)
  	.p2align	2          # Causes a new fragment to be emitted

  	.type	other_function,@function
  other_function:
  	ret

exposes an odd behaviour in which only the %pcrel_hi relocation is
evaluated but not the %pcrel_lo.

  $ llvm-mc -triple riscv64 -filetype obj t.s | llvm-objdump  -d -r -

  <stdin>:	file format ELF64-riscv

  Disassembly of section .text:
  0000000000000000 function:
         0:	17 05 00 00	auipc	a0, 0
         4:	93 05 05 00	mv	a1, a0
  		0000000000000004:  R_RISCV_PCREL_LO12_I	other_function+4

  0000000000000008 other_function:
         8:	67 80 00 00	ret

The reason seems to be that in RISCVAsmBackend::shouldForceRelocation we
only consider the fragment but in RISCVMCExpr::evaluatePCRelLo we
consider the section. This usually works but there are cases where the
section may still be the same but the fragment may be another one. In
that case we end forcing a %pcrel_lo relocation without any %pcrel_hi.

This patch makes RISCVAsmBackend::shouldForceRelocation use the section,
if any, to determine if the relocation must be forced or not.

Differential Revision: https://reviews.llvm.org/D60657

llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp

@@ -64,11 +64,15 @@ bool RISCVAsmBackend::shouldForceRelocation(const MCAssembler &Asm,
     case RISCV::fixup_riscv_tls_gd_hi20:
       ShouldForce = true;
       break;
-    case RISCV::fixup_riscv_pcrel_hi20:
-      ShouldForce = T->getValue()->findAssociatedFragment() !=
-                    Fixup.getValue()->findAssociatedFragment();
+    case RISCV::fixup_riscv_pcrel_hi20: {
+      MCFragment *TFragment = T->getValue()->findAssociatedFragment();
+      MCFragment *FixupFragment = Fixup.getValue()->findAssociatedFragment();
+      assert(FixupFragment && "We should have a fragment for this fixup");
+      ShouldForce =
+          !TFragment || TFragment->getParent() != FixupFragment->getParent();
       break;
     }
+    }
     break;
   }
 

llvm/test/MC/RISCV/pcrel-fixups.s

@@ -0,0 +1,52 @@
+# RUN: llvm-mc -triple riscv32 -mattr=-relax -filetype obj %s \
+# RUN:    | llvm-objdump -M no-aliases -d -r - \
+# RUN:    | FileCheck --check-prefix NORELAX %s
+# RUN: llvm-mc -triple riscv32 -mattr=+relax -filetype obj %s \
+# RUN:    | llvm-objdump -M no-aliases -d -r - \
+# RUN:    | FileCheck --check-prefix RELAX %s
+# RUN: llvm-mc -triple riscv64 -mattr=-relax -filetype obj %s \
+# RUN:    | llvm-objdump -M no-aliases -d -r - \
+# RUN:    | FileCheck --check-prefix NORELAX %s
+# RUN: llvm-mc -triple riscv64 -mattr=+relax -filetype obj %s \
+# RUN:    | llvm-objdump -M no-aliases -d -r - \
+# RUN:    | FileCheck --check-prefix RELAX %s
+
+# Fixups for %pcrel_hi / %pcrel_lo can be evaluated within a section,
+# regardless of the fragment containing the target address.
+
+function:
+.Lpcrel_label1:
+	auipc	a0, %pcrel_hi(other_function)
+	addi	a1, a0, %pcrel_lo(.Lpcrel_label1)
+# NORELAX: auipc	a0, 0
+# NORELAX-NOT: R_RISCV
+# NORELAX: addi	a1, a0, 16
+# NORELAX-NOT: R_RISCV
+
+# RELAX: auipc	a0, 0
+# RELAX: R_RISCV_PCREL_HI20	other_function
+# RELAX: R_RISCV_RELAX	*ABS*
+# RELAX: addi	a1, a0, 0
+# RELAX: R_RISCV_PCREL_LO12_I	.Lpcrel_label1
+# RELAX: R_RISCV_RELAX	*ABS*
+
+	.p2align	2   # Cause a new fragment be emitted here
+.Lpcrel_label2:
+	auipc	a0, %pcrel_hi(other_function)
+	addi	a1, a0, %pcrel_lo(.Lpcrel_label2)
+# NORELAX: auipc	a0, 0
+# NORELAX-NOT: R_RISCV
+# NORELAX: addi	a1, a0, 8
+# NORELAX-NOT: R_RISCV
+
+# RELAX: auipc	a0, 0
+# RELAX: R_RISCV_PCREL_HI20	other_function
+# RELAX: R_RISCV_RELAX	*ABS*
+# RELAX: addi	a1, a0, 0
+# RELAX: R_RISCV_PCREL_LO12_I	.Lpcrel_label2
+# RELAX: R_RISCV_RELAX	*ABS*
+
+	.type	other_function,@function
+other_function:
+	ret
+