[CHERIoT] Fold csetboundsexact to csetboundsimm when the bounds is a constant less than 512.

This is guaranteed always to be exact, and it saves materializing the constant.

Author: resistor

llvm/lib/Target/RISCV/RISCVInstrInfoXCheri.td

@@ -1450,6 +1450,14 @@ def : PatGpcrGpcr<int_cheri_cap_type_copy, CCopyType>;
 def : PatGpcrGpcr<int_cheri_cap_conditional_seal, CCSeal>;
 def : PatGpcr<int_cheri_cap_seal_entry, CSealEntry, CLenVT>;
 
+def uimm12_lt512 : Operand<XLenVT>,
+                   ImmLeaf<XLenVT, "return isUInt<12>(Imm) && (Imm < 512);">;
+
+let Predicates =
+    [HasCheriot] in def : Pat<(int_cheri_cap_bounds_set_exact GPCR:$rs1,
+                                  uimm12_lt512:$imm12),
+                              (CSetBoundsImm GPCR:$rs1, uimm12_lt512:$imm12)>;
+
 // TODO: Make this rematerialisable like MIPS
 def : PatGpcrGpr<int_cheri_bounded_stack_cap, CSetBounds>;
 def : PatGpcrUimm12<int_cheri_bounded_stack_cap, CSetBoundsImm>;

llvm/test/CodeGen/RISCV/cheri/cheriot-csetboundsexact.ll

@@ -0,0 +1,32 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple=riscv32cheriot --mcpu=cheriot -target-abi=cheriot -mattr=+xcheri,+cap-mode,+xcheriot -o - %s | FileCheck %s
+
+define i8 addrspace(200)* @test1(i8 addrspace(200)* %cap, i32 %bounds) nounwind {
+; CHECK-LABEL: test1:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    ct.csetbounds ca0, ca0, 3
+; CHECK-NEXT:    ct.cret
+  %newcap = call i8 addrspace(200)* @llvm.cheri.cap.bounds.set.exact.i32(i8 addrspace(200)* %cap, i32 3)
+  ret i8 addrspace(200)* %newcap
+}
+
+define i8 addrspace(200)* @test2(i8 addrspace(200)* %cap, i32 %bounds) nounwind {
+; CHECK-LABEL: test2:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    ct.csetbounds ca0, ca0, 511
+; CHECK-NEXT:    ct.cret
+  %newcap = call i8 addrspace(200)* @llvm.cheri.cap.bounds.set.exact.i32(i8 addrspace(200)* %cap, i32 511)
+  ret i8 addrspace(200)* %newcap
+}
+
+define i8 addrspace(200)* @test3(i8 addrspace(200)* %cap, i32 %bounds) nounwind {
+; CHECK-LABEL: test3:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    li a1, 512
+; CHECK-NEXT:    ct.csetboundsexact ca0, ca0, a1
+; CHECK-NEXT:    ct.cret
+  %newcap = call i8 addrspace(200)* @llvm.cheri.cap.bounds.set.exact.i32(i8 addrspace(200)* %cap, i32 512)
+  ret i8 addrspace(200)* %newcap
+}
+
+declare i8 addrspace(200)* @llvm.cheri.cap.bounds.set.exact.i32(i8 addrspace(200)*, i32)