[RISCV] Lower the alignment requirement for a GPR pair spill for Zdinx on RV32. (#85871)

I believe we can use XLen alignment as long as eliminateFrameIndex
limits the maximum folded offset to 2043. This way when we split
the load/store into two 2 instructions we'll be able to add 4
without overflowing simm12.

Author: topperc

llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp

@@ -446,6 +446,13 @@ bool RISCVRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
                (Lo12 & 0b11111) != 0) {
       // Prefetch instructions require the offset to be 32 byte aligned.
       MI.getOperand(FIOperandNum + 1).ChangeToImmediate(0);
+    } else if ((Opc == RISCV::PseudoRV32ZdinxLD ||
+                Opc == RISCV::PseudoRV32ZdinxSD) &&
+               Lo12 >= 2044) {
+      // This instruction will be split into 2 instructions. The second
+      // instruction will add 4 to the immediate. If that would overflow 12
+      // bits, we can't fold the offset.
+      MI.getOperand(FIOperandNum + 1).ChangeToImmediate(0);
     } else {
       // We can encode an add with 12 bit signed immediate in the immediate
       // operand of our user instruction.  As a result, the remaining

llvm/lib/Target/RISCV/RISCVRegisterInfo.td

@@ -573,7 +573,7 @@ let RegAltNameIndices = [ABIRegAltName] in {
 }
 
 let RegInfos = RegInfoByHwMode<[RV32, RV64],
-                               [RegInfo<64, 64, 64>, RegInfo<128, 128, 128>]>,
+                               [RegInfo<64, 64, 32>, RegInfo<128, 128, 64>]>,
     DecoderMethod = "DecodeGPRPairRegisterClass" in
 def GPRPair : RegisterClass<"RISCV", [XLenPairFVT], 64, (add
     X10_X11, X12_X13, X14_X15, X16_X17,

llvm/test/CodeGen/RISCV/zdinx-large-spill.mir

@@ -0,0 +1,74 @@
+# NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 4
+# RUN: llc %s -mtriple=riscv32 -mattr=+zdinx -start-before=prologepilog -o - | FileCheck %s
+
+# We want to make sure eliminateFrameIndex doesn't fold sp+2044 as an offset in
+# a GPR pair spill/reload instruction. When we split the pair spill, we would be
+# unable to add 4 to the immediate without overflowing simm12.
+
+--- |
+  define void @foo() {
+  ; CHECK-LABEL: foo:
+  ; CHECK:       # %bb.0:
+  ; CHECK-NEXT:    addi sp, sp, -2048
+  ; CHECK-NEXT:    addi sp, sp, -16
+  ; CHECK-NEXT:    .cfi_def_cfa_offset 2064
+  ; CHECK-NEXT:    lui t0, 1
+  ; CHECK-NEXT:    add t0, sp, t0
+  ; CHECK-NEXT:    sw a0, -2040(t0)
+  ; CHECK-NEXT:    sw a1, -2036(t0)
+  ; CHECK-NEXT:    lui a0, 1
+  ; CHECK-NEXT:    add a0, sp, a0
+  ; CHECK-NEXT:    sw a2, -2048(a0)
+  ; CHECK-NEXT:    sw a3, -2044(a0)
+  ; CHECK-NEXT:    sw a4, 2040(sp)
+  ; CHECK-NEXT:    sw a5, 2044(sp)
+  ; CHECK-NEXT:    sw a6, 2032(sp)
+  ; CHECK-NEXT:    sw a7, 2036(sp)
+  ; CHECK-NEXT:    lui a0, 1
+  ; CHECK-NEXT:    add a0, sp, a0
+  ; CHECK-NEXT:    lw a1, -2036(a0)
+  ; CHECK-NEXT:    lw a0, -2040(a0)
+  ; CHECK-NEXT:    lui a0, 1
+  ; CHECK-NEXT:    add a0, sp, a0
+  ; CHECK-NEXT:    lw a2, -2048(a0)
+  ; CHECK-NEXT:    lw a3, -2044(a0)
+  ; CHECK-NEXT:    lw a4, 2040(sp)
+  ; CHECK-NEXT:    lw a5, 2044(sp)
+  ; CHECK-NEXT:    lw a6, 2032(sp)
+  ; CHECK-NEXT:    lw a7, 2036(sp)
+  ; CHECK-NEXT:    addi sp, sp, 2032
+  ; CHECK-NEXT:    addi sp, sp, 32
+  ; CHECK-NEXT:    ret
+    ret void
+  }
+...
+---
+name:            foo
+tracksRegLiveness: true
+tracksDebugUserValues: true
+frameInfo:
+  maxAlignment:    4
+stack:
+  - { id: 0, type: spill-slot, size: 8, alignment: 4 }
+  - { id: 1, type: spill-slot, size: 8, alignment: 4 }
+  - { id: 2, type: spill-slot, size: 8, alignment: 4 }
+  - { id: 3, type: spill-slot, size: 8, alignment: 4 }
+  - { id: 4, type: spill-slot, size: 2024, alignment: 4 }
+machineFunctionInfo:
+  varArgsFrameIndex: 0
+  varArgsSaveSize: 0
+body:             |
+  bb.0:
+    liveins: $x10_x11, $x12_x13, $x14_x15, $x16_x17
+
+    PseudoRV32ZdinxSD killed renamable $x10_x11, %stack.0, 0 :: (store (s64) into %stack.0, align 4)
+    PseudoRV32ZdinxSD killed renamable $x12_x13, %stack.1, 0 :: (store (s64) into %stack.1, align 4)
+    PseudoRV32ZdinxSD killed renamable $x14_x15, %stack.2, 0 :: (store (s64) into %stack.2, align 4)
+    PseudoRV32ZdinxSD killed renamable $x16_x17, %stack.3, 0 :: (store (s64) into %stack.3, align 4)
+    renamable $x10_x11 = PseudoRV32ZdinxLD %stack.0, 0 :: (load (s64) from %stack.0, align 4)
+    renamable $x12_x13 = PseudoRV32ZdinxLD %stack.1, 0 :: (load (s64) from %stack.1, align 4)
+    renamable $x14_x15 = PseudoRV32ZdinxLD %stack.2, 0 :: (load (s64) from %stack.2, align 4)
+    renamable $x16_x17 = PseudoRV32ZdinxLD %stack.3, 0 :: (load (s64) from %stack.3, align 4)
+    PseudoRET
+
+...