diff --git a/llvm/lib/Target/AArch64/AArch64InstrInfo.td b/llvm/lib/Target/AArch64/AArch64InstrInfo.td
index 76f076a60765f..b30e3d06b2c9f 100644
--- a/llvm/lib/Target/AArch64/AArch64InstrInfo.td
+++ b/llvm/lib/Target/AArch64/AArch64InstrInfo.td
@@ -4444,6 +4444,11 @@ defm PRFUM : PrefetchUnscaled<0b11, 0, 0b10, "prfum",
                   [(AArch64Prefetch timm:$Rt,
                                   (am_unscaled64 GPR64sp:$Rn, simm9:$offset))]>;
 
+// PRFM falls back to PRFUM for negative or unaligned offsets (not a multiple
+// of 8).
+def : InstAlias<"prfm $Rt, [$Rn, $offset]",
+                (PRFUMi prfop:$Rt, GPR64sp:$Rn, simm9_offset_fb64:$offset), 0>;
+
 //---
 // (unscaled immediate, unprivileged)
 defm LDTRX : LoadUnprivileged<0b11, 0, 0b01, GPR64, "ldtr">;
diff --git a/llvm/test/MC/AArch64/prfum.s b/llvm/test/MC/AArch64/prfum.s
new file mode 100644
index 0000000000000..81a864a694325
--- /dev/null
+++ b/llvm/test/MC/AArch64/prfum.s
@@ -0,0 +1,44 @@
+// RUN: llvm-mc -triple=aarch64 -show-encoding --print-imm-hex=false < %s \
+// RUN:        | FileCheck %s --check-prefixes=CHECK-ENCODING,CHECK-INST
+// RUN: llvm-mc -triple=aarch64 -filetype=obj < %s \
+// RUN:        | llvm-objdump -d --print-imm-hex=false - | FileCheck %s --check-prefix=CHECK-INST
+// Disassemble encoding and check the re-encoding (-show-encoding) matches.
+// RUN: llvm-mc -triple=aarch64 -show-encoding < %s \
+// RUN:        | sed '/.text/d' | sed 's/.*encoding: //g' \
+// RUN:        | llvm-mc -triple=aarch64 -disassemble -show-encoding --print-imm-hex=false \
+// RUN:        | FileCheck %s --check-prefixes=CHECK-ENCODING,CHECK-INST
+
+// PRFM falls back to PRFUM for negative or unaligned offsets (not a multiple
+// of 8).
+
+prfm pldl1keep, [x0, #-256]
+// CHECK-INST: prfum pldl1keep, [x0, #-256]
+// CHECK-ENCODING: [0x00,0x00,0x90,0xf8]
+
+prfm pldl1keep, [x0, #-8]
+// CHECK-INST: prfum pldl1keep, [x0, #-8]
+// CHECK-ENCODING: [0x00,0x80,0x9f,0xf8]
+
+prfm pldl1keep, [x0, #-1]
+// CHECK-INST: prfum pldl1keep, [x0, #-1]
+// CHECK-ENCODING: [0x00,0xf0,0x9f,0xf8]
+
+prfm pldl1keep, [x0, #0]
+// CHECK-INST: prfm pldl1keep, [x0]
+// CHECK-ENCODING: [0x00,0x00,0x80,0xf9]
+
+prfm pldl1keep, [x0, #1]
+// CHECK-INST: prfum pldl1keep, [x0, #1]
+// CHECK-ENCODING: [0x00,0x10,0x80,0xf8]
+
+prfm pldl1keep, [x0, #8]
+// CHECK-INST: prfm pldl1keep, [x0, #8]
+// CHECK-ENCODING: [0x00,0x04,0x80,0xf9]
+
+prfm pldl1keep, [x0, #255]
+// CHECK-INST: prfum pldl1keep, [x0, #255]
+// CHECK-ENCODING: [0x00,0xf0,0x8f,0xf8]
+
+prfm pldl1keep, [x0, #256]
+// CHECK-INST: prfm pldl1keep, [x0, #256]
+// CHECK-ENCODING: [0x00,0x80,0x80,0xf9]