[RISCV] Don't evaluatePCRelLo if a relocation will be forced (e.g. due to linker relaxation)

[asb]

Summary

A pcrel_lo will point to the associated pcrel_hi fixup which in turn points to the real target. RISCVMCExpr::evaluatePCRelLo will work around this indirection in order to allow the fixup to be evaluate properly. However, if relocations are forced (e.g. due to linker relaxation is enabled) then its evaluation is undesired and will result in a relocation with the wrong target.

This patch modifies evaluatePCRelLo so it will not try to evaluate if the fixup will be forced as a relocation. A new helper method is added to RISCVAsmBackend to query this.

[shiva0217]

Could you add a case in option-relax.s after .option norelax to indicate the relocation type for local symbol will leave once the relaxation has been enabled?

[arunthomas]

Thanks for fixing this issue, Alex. Looking forward to seeing this merged!

[asb]

Thanks for the review @shiva0217 - how is this?

[shiva0217]

Thanks for the update, LGTM.

lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h

RISCVAsmBackend(const MCSubtargetInfo &STI, uint8_t OSABI, bool Is64Bit,
         TargetOptions(Options) {
     TargetABI = RISCVABI::computeTargetABI(
         STI.getTargetTriple(), STI.getFeatureBits(), Options.getABIName());
   }
   ~RISCVAsmBackend() override {}
 
   void setForceRelocs() { ForceRelocs = true; }
 
+  // Returns true if relocations will be forced for shouldForceRelocation by
+  // default. This will be true if relaxation is enabled or had previously
+  // been enabled.
+  bool willForceRelocations() const {
+    return ForceRelocs || STI.getFeatureBits()[RISCV::FeatureRelax];
+  }
+
   // Generate diff expression relocations if the relax feature is enabled or had
   // previously been enabled, otherwise it is safe for the assembler to
   // calculate these internally.
   bool requiresDiffExpressionRelocations() const override {
-    return STI.getFeatureBits()[RISCV::FeatureRelax] || ForceRelocs;
+    return willForceRelocations();
   }
 
   // Return Size with extra Nop Bytes for alignment directive in code section.
   bool shouldInsertExtraNopBytesForCodeAlign(const MCAlignFragment &AF,
                                              unsigned &Size) override;
 
   // Insert target specific fixup type for alignment directive in code section.
   bool shouldInsertFixupForCodeAlign(MCAssembler &Asm,

lib/Target/RISCV/MCTargetDesc/RISCVMCExpr.cpp

 //===-- RISCVMCExpr.cpp - RISCV specific MC expression classes ------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file contains the implementation of the assembly expression modifiers
 // accepted by the RISCV architecture (e.g. ":lo12:", ":gottprel_g1:", ...).
 //
 //===----------------------------------------------------------------------===//
 
-#include "RISCV.h"
 #include "RISCVMCExpr.h"
+#include "MCTargetDesc/RISCVAsmBackend.h"
+#include "RISCV.h"
 #include "RISCVFixupKinds.h"
+#include "llvm/MC/MCAsmLayout.h"
 #include "llvm/MC/MCAssembler.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCSymbolELF.h"
 #include "llvm/MC/MCValue.h"
 #include "llvm/Support/ErrorHandling.h"
 
 using namespace llvm;

 bool RISCVMCExpr::evaluatePCRelLo(MCValue &Res, const MCAsmLayout *Layout,
                                   const MCFixup *Fixup) const {
   // VK_RISCV_PCREL_LO has to be handled specially.  The MCExpr inside is
   // actually the location of a auipc instruction with a VK_RISCV_PCREL_HI fixup
   // pointing to the real target.  We need to generate an MCValue in the form of
   // (<real target> + <offset from this fixup to the auipc fixup>).  The Fixup
   // is pcrel relative to the VK_RISCV_PCREL_LO fixup, so we need to add the
   // offset to the VK_RISCV_PCREL_HI Fixup from VK_RISCV_PCREL_LO to correct.
+
+  // Don't try to evaluate if the fixup will be forced as a relocation (e.g.
+  // as linker relaxation is enabled). If we evaluated pcrel_lo in this case,
+  // the modified fixup will be converted into a relocation that no longer
+  // points to the pcrel_hi as the linker requires.
+  auto &RAB =
+      static_cast<RISCVAsmBackend &>(Layout->getAssembler().getBackend());
+  if (RAB.willForceRelocations())
+    return false;
+
   MCValue AUIPCLoc;
   if (!getSubExpr()->evaluateAsValue(AUIPCLoc, *Layout))
     return false;
 
   const MCSymbolRefExpr *AUIPCSRE = AUIPCLoc.getSymA();
   // Don't try to evaluate %pcrel_hi/%pcrel_lo pairs that cross fragment
   // boundries.
   if (!AUIPCSRE ||

test/MC/RISCV/linker-relaxation.s

 auipc t1, %pcrel_hi(bar)
 # NORELAX-RELOC-NOT: R_RISCV_PCREL_HI20
 # NORELAX-RELOC-NOT: R_RISCV_RELAX
 # RELAX-RELOC: R_RISCV_PCREL_HI20 bar 0x0
 # RELAX-RELOC: R_RISCV_RELAX - 0x0
 # RELAX-FIXUP: fixup A - offset: 0, value: %pcrel_hi(bar), kind: fixup_riscv_pcrel_hi20
 # RELAX-FIXUP: fixup B - offset: 0, value: 0, kind: fixup_riscv_relax
 
-# TODO/FIXME: The generated PCREL_LO relocations are incorrect.
-# RISCVMCExpr::evaluatePCRelLo should not be evaluating the fixup when linker
-# relaxation is enabled.
-
 addi t1, t1, %pcrel_lo(2b)
 # NORELAX-RELOC-NOT: R_RISCV_PCREL_LO12_I
 # NORELAX-RELOC-NOT: R_RISCV_RELAX
-# RELAX-RELOC: R_RISCV_PCREL_LO12_I bar 0x4
+# RELAX-RELOC: R_RISCV_PCREL_LO12_I .Ltmp1 0x0
 # RELAX-RELOC: R_RISCV_RELAX - 0x0
 # RELAX-FIXUP: fixup A - offset: 0, value: %pcrel_lo(.Ltmp1), kind: fixup_riscv_pcrel_lo12_i
 # RELAX-FIXUP: fixup B - offset: 0, value: 0, kind: fixup_riscv_relax
 
 sb t1, %pcrel_lo(2b)(a2)
 # NORELAX-RELOC-NOT: R_RISCV_PCREL_LO12_S
 # NORELAX-RELOC-NOT: R_RISCV_RELAX
-# RELAX-RELOC: R_RISCV_PCREL_LO12_S bar 0x8
+# RELAX-RELOC: R_RISCV_PCREL_LO12_S .Ltmp1 0x0
 # RELAX-RELOC: R_RISCV_RELAX - 0x0
 # RELAX-FIXUP: fixup A - offset: 0, value: %pcrel_lo(.Ltmp1), kind: fixup_riscv_pcrel_lo12_s
 # RELAX-FIXUP: fixup B - offset: 0, value: 0, kind: fixup_riscv_relax