[RISCV] Fix wrong register rename for store value during make-compressible optimization

Current implementation will rename both register in store instructions if
we store base address into memory with same base register, it's OK if
the offset is 0, however that is wrong transform if offset isn't 0, give
a smalle example here:

sd      a0, 808(a0)

We should not transform into:

addi    a2, a0, 768
sd      a2, 40(a2)

That should just rename base address like this:

addi    a2, a0, 768
sd      a0, 40(a2)

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D128876

Author: kito-cheng

llvm/lib/Target/RISCV/RISCVMakeCompressible.cpp

@@ -293,8 +293,16 @@ static void updateOperands(MachineInstr &MI, RegImmPair OldRegImm,
   assert((isCompressibleLoad(MI) || isCompressibleStore(MI)) &&
          "Unsupported instruction for this optimization.");
 
+  int SkipN = 0;
+
+  // Skip the first (value) operand to a store instruction (except if the store
+  // offset is zero) in order to avoid an incorrect transformation.
+  // e.g. sd a0, 808(a0) to addi a2, a0, 768; sd a2, 40(a2)
+  if (isCompressibleStore(MI) && OldRegImm.Imm != 0)
+    SkipN = 1;
+
   // Update registers
-  for (MachineOperand &MO : MI.operands())
+  for (MachineOperand &MO : drop_begin(MI.operands(), SkipN))
     if (MO.isReg() && MO.getReg() == OldRegImm.Reg) {
       // Do not update operands that define the old register.
       //

llvm/test/CodeGen/RISCV/make-compressible-for-store-address.mir

@@ -33,7 +33,7 @@ body:             |
     ; CHECK-NEXT: renamable $x11 = ADDI $x0, 1
     ; CHECK-NEXT: $x12 = ADDI $x10, 768
     ; CHECK-NEXT: SD killed renamable $x11, $x12, 32 :: (store (s64) into %ir.1)
-    ; CHECK-NEXT: SD $x12, $x12, 40 :: (store (s64) into %ir.2)
+    ; CHECK-NEXT: SD renamable $x10, $x12, 40 :: (store (s64) into %ir.2)
     ; CHECK-NEXT: renamable $x11 = ADDI $x0, 2
     ; CHECK-NEXT: SD killed renamable $x11, killed $x12, 48 :: (store (s64) into %ir.3)
     ; CHECK-NEXT: PseudoRET