[AArch64] Fix FPMR handling when switching streaming mode (llvm#135827)

According to the
[documentation](https://developer.arm.com/documentation/ddi0601/latest/AArch64-Registers/FPMR--Floating-point-Mode-Register),
the FPMR register is set to 0 when entering or exiting streaming mode.
This patch models that behavior by adding FPMR as an implicit def to the
instructions used for entering and exiting streaming mode.

Author: Lukacma

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -8882,12 +8882,15 @@ void AArch64TargetLowering::AdjustInstrPostInstrSelection(MachineInstr &MI,
         MI.removeOperand(I);
 
     // The SVE vector length can change when entering/leaving streaming mode.
+    // FPMR is set to 0 when entering/leaving streaming mode.
     if (MI.getOperand(0).getImm() == AArch64SVCR::SVCRSM ||
         MI.getOperand(0).getImm() == AArch64SVCR::SVCRSMZA) {
       MI.addOperand(MachineOperand::CreateReg(AArch64::VG, /*IsDef=*/false,
                                               /*IsImplicit=*/true));
       MI.addOperand(MachineOperand::CreateReg(AArch64::VG, /*IsDef=*/true,
                                               /*IsImplicit=*/true));
+      MI.addOperand(MachineOperand::CreateReg(AArch64::FPMR, /*IsDef=*/true,
+                                              /*IsImplicit=*/true));
     }
   }
 

llvm/test/CodeGen/AArch64/sme-write-fpmr.ll

@@ -0,0 +1,23 @@
+; RUN: llc -mattr=+sme -stop-after=finalize-isel < %s | FileCheck %s
+
+target triple = "aarch64"
+
+; Check that we don't define FPMR for 'smstart za' and 'smstop za'
+define void @smstart_za() "aarch64_new_za" nounwind {
+  ; CHECK-LABEL:    name: smstart_za
+  ; CHECK-NOT:        implicit-def {{[^,]*}}$fpmr
+  ret void
+}
+
+; Check that we do define FPMR for 'smstart sm' and 'smstop sm'
+define void @smstart_sm() nounwind {
+  ; CHECK-LABEL: name: smstart_sm
+  ; CHECK:          MSRpstatesvcrImm1 1, 1,
+  ; CHECK-SAME:       implicit-def {{[^,]*}}$fpmr
+  ; CHECK:          MSRpstatesvcrImm1 1, 0,
+  ; CHECK-SAME:       implicit-def {{[^,]*}}$fpmr
+  call void @require_sm()
+  ret void
+}
+
+declare void @require_sm() "aarch64_pstate_sm_enabled"