Allow negative offsets in MipsMCInstLower::LowerOperand

Summary:
We rely on this in our CHERI backend to address the GOT by generating a
$pc-relative addresses. For this we emit the following code sequence:

lui $1, %pcrel_hi(_CHERI_CAPABILITY_TABLE_-8)
daddiu $1, $1, %pcrel_lo(_CHERI_CAPABILITY_TABLE_-4)
cgetpccincoffset $c1, $1

However, without this change the addend is implicitly converted to
UINT32_MAX and an invalid pointer value is generated.

Reviewers: atanasyan

Reviewed By: atanasyan

Subscribers: merge_guards_bot, sdardis, hiraditya, jrtc27, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70953

2 files changed, 5 insertions, 7 deletions

diff --git a/llvm/lib/Target/Mips/MipsMCInstLower.cpp b/llvm/lib/Target/Mips/MipsMCInstLower.cpp
index fd984058a2b..66e04bda2af 100644
--- a/llvm/lib/Target/Mips/MipsMCInstLower.cpp
+++ b/llvm/lib/Target/Mips/MipsMCInstLower.cpp
@@ -34,7 +34,7 @@ void MipsMCInstLower::Initialize(MCContext *C) {
 
 MCOperand MipsMCInstLower::LowerSymbolOperand(const MachineOperand &MO,
                                               MachineOperandType MOTy,
-                                              unsigned Offset) const {
+                                              int64_t Offset) const {
   MCSymbolRefExpr::VariantKind Kind = MCSymbolRefExpr::VK_None;
   MipsMCExpr::MipsExprKind TargetKind = MipsMCExpr::MEK_None;
   bool IsGpOff = false;
@@ -161,9 +161,7 @@ MCOperand MipsMCInstLower::LowerSymbolOperand(const MachineOperand &MO,
   const MCExpr *Expr = MCSymbolRefExpr::create(Symbol, Kind, *Ctx);
 
   if (Offset) {
-    // Assume offset is never negative.
-    assert(Offset > 0);
-
+    // Note: Offset can also be negative
     Expr = MCBinaryExpr::createAdd(Expr, MCConstantExpr::create(Offset, *Ctx),
                                    *Ctx);
   }
@@ -177,7 +175,7 @@ MCOperand MipsMCInstLower::LowerSymbolOperand(const MachineOperand &MO,
 }
 
 MCOperand MipsMCInstLower::LowerOperand(const MachineOperand &MO,
-                                        unsigned offset) const {
+                                        int64_t offset) const {
   MachineOperandType MOTy = MO.getType();
 
   switch (MOTy) {
diff --git a/llvm/lib/Target/Mips/MipsMCInstLower.h b/llvm/lib/Target/Mips/MipsMCInstLower.h
index 29af6f21de8..605a124bf10 100644
--- a/llvm/lib/Target/Mips/MipsMCInstLower.h
+++ b/llvm/lib/Target/Mips/MipsMCInstLower.h
@@ -35,11 +35,11 @@ public:
 
   void Initialize(MCContext *C);
   void Lower(const MachineInstr *MI, MCInst &OutMI) const;
-  MCOperand LowerOperand(const MachineOperand& MO, unsigned offset = 0) const;
+  MCOperand LowerOperand(const MachineOperand &MO, int64_t offset = 0) const;
 
 private:
   MCOperand LowerSymbolOperand(const MachineOperand &MO,
-                               MachineOperandType MOTy, unsigned Offset) const;
+                               MachineOperandType MOTy, int64_t Offset) const;
   MCOperand createSub(MachineBasicBlock *BB1, MachineBasicBlock *BB2,
                       MipsMCExpr::MipsExprKind Kind) const;
   void lowerLongBranchLUi(const MachineInstr *MI, MCInst &OutMI) const;