[RISCV] Add WasForced parameter to MCAsmBackend::fixupNeedsRelaxationAdvanced

For RISCV branch instructions, we need to preserve relocation types when linker
relaxation enabled, so then linker could modify offset when the branch offsets
changed.

We preserve relocation types by define shouldForceRelocation.
IsResolved return by evaluateFixup will always false when shouldForceRelocation
return true. It will make RISCV MC Branch Relaxation always relax 16-bit
branches to 32-bit form, even if the symbol actually could be resolved.

To avoid 16-bit branches always relax to 32-bit form when linker relaxation
enabled, we add a new parameter WasForced to indicate that the symbol actually
couldn't be resolved and not forced by shouldForceRelocation return true.

RISCVAsmBackend::fixupNeedsRelaxationAdvanced could relax branches with
unresolved symbols by (!IsResolved && !WasForced).

RISCV MC Branch Relaxation is needed because RISCV could perform 32-bit
to 16-bit transformation in MC layer.

Differential Revision: https://reviews.llvm.org/D46350

llvm-svn: 332696

2 files changed, 15 insertions, 7 deletions

diff --git a/llvm/lib/MC/MCAsmBackend.cpp b/llvm/lib/MC/MCAsmBackend.cpp
index b4a4d0a8996..d675a98542f 100644
--- a/llvm/lib/MC/MCAsmBackend.cpp
+++ b/llvm/lib/MC/MCAsmBackend.cpp
@@ -58,7 +58,8 @@ const MCFixupKindInfo &MCAsmBackend::getFixupKindInfo(MCFixupKind Kind) const {
 
 bool MCAsmBackend::fixupNeedsRelaxationAdvanced(
     const MCFixup &Fixup, bool Resolved, uint64_t Value,
-    const MCRelaxableFragment *DF, const MCAsmLayout &Layout) const {
+    const MCRelaxableFragment *DF, const MCAsmLayout &Layout,
+    const bool WasForced) const {
   if (!Resolved)
     return true;
   return fixupNeedsRelaxation(Fixup, Value, DF, Layout);
@@ -84,4 +85,4 @@ void MCAsmBackend::handleCodePaddingInstructionEnd(const MCInst &Inst) {
 
 bool MCAsmBackend::relaxFragment(MCPaddingFragment *PF, MCAsmLayout &Layout) {
   return CodePadder->relaxFragment(PF, Layout);
-}
\ No newline at end of file
+}
diff --git a/llvm/lib/MC/MCAssembler.cpp b/llvm/lib/MC/MCAssembler.cpp
index b6af11d09ba..a8a0177dff6 100644
--- a/llvm/lib/MC/MCAssembler.cpp
+++ b/llvm/lib/MC/MCAssembler.cpp
@@ -197,7 +197,8 @@ const MCSymbol *MCAssembler::getAtom(const MCSymbol &S) const {
 
 bool MCAssembler::evaluateFixup(const MCAsmLayout &Layout,
                                 const MCFixup &Fixup, const MCFragment *DF,
-                                MCValue &Target, uint64_t &Value) const {
+                                MCValue &Target, uint64_t &Value,
+                                bool &WasForced) const {
   ++stats::evaluateFixup;
 
   // FIXME: This code has some duplication with recordRelocation. We should
@@ -209,6 +210,7 @@ bool MCAssembler::evaluateFixup(const MCAsmLayout &Layout,
   const MCExpr *Expr = Fixup.getValue();
   MCContext &Ctx = getContext();
   Value = 0;
+  WasForced = false;
   if (!Expr->evaluateAsRelocatable(Target, &Layout, &Fixup)) {
     Ctx.reportError(Fixup.getLoc(), "expected relocatable expression");
     return true;
@@ -273,8 +275,10 @@ bool MCAssembler::evaluateFixup(const MCAsmLayout &Layout,
   }
 
   // Let the backend force a relocation if needed.
-  if (IsResolved && getBackend().shouldForceRelocation(*this, Fixup, Target))
+  if (IsResolved && getBackend().shouldForceRelocation(*this, Fixup, Target)) {
     IsResolved = false;
+    WasForced = true;
+  }
 
   return IsResolved;
 }
@@ -689,7 +693,9 @@ MCAssembler::handleFixup(const MCAsmLayout &Layout, MCFragment &F,
   // Evaluate the fixup.
   MCValue Target;
   uint64_t FixedValue;
-  bool IsResolved = evaluateFixup(Layout, Fixup, &F, Target, FixedValue);
+  bool WasForced;
+  bool IsResolved = evaluateFixup(Layout, Fixup, &F, Target, FixedValue,
+                                  WasForced);
   if (!IsResolved) {
     // The fixup was unresolved, we need a relocation. Inform the object
     // writer of the relocation, and give it an opportunity to adjust the
@@ -804,13 +810,14 @@ bool MCAssembler::fixupNeedsRelaxation(const MCFixup &Fixup,
   assert(getBackendPtr() && "Expected assembler backend");
   MCValue Target;
   uint64_t Value;
-  bool Resolved = evaluateFixup(Layout, Fixup, DF, Target, Value);
+  bool WasForced;
+  bool Resolved = evaluateFixup(Layout, Fixup, DF, Target, Value, WasForced);
   if (Target.getSymA() &&
       Target.getSymA()->getKind() == MCSymbolRefExpr::VK_X86_ABS8 &&
       Fixup.getKind() == FK_Data_1)
     return false;
   return getBackend().fixupNeedsRelaxationAdvanced(Fixup, Resolved, Value, DF,
-                                                   Layout);
+                                                   Layout, WasForced);
 }
 
 bool MCAssembler::fragmentNeedsRelaxation(const MCRelaxableFragment *F,