Remove uses of reg_mod from s390x (#5073)

Remove uses of reg_mod from the s390x backend. This required moving away from using r0/r1 as the result registers from a few different pseudo instructions, standardizing instead on r2/r3. That change was necessary as regalloc2 will not correctly allocate registers that aren't listed in the allocatable set, which r0/r1 are not.

Co-authored-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>
Co-authored-by: Chris Fallin <chris@cfallin.org>

cranelift/codegen/src/isa/s390x/inst/regs.rs

@@ -5,6 +5,7 @@ use regalloc2::MachineEnv;
 use regalloc2::PReg;
 use regalloc2::VReg;
 
+use crate::isa::s390x::inst::{RegPair, WritableRegPair};
 use crate::machinst::*;
 use crate::settings;
 
@@ -178,6 +179,24 @@ pub fn pretty_print_reg(reg: Reg, allocs: &mut AllocationConsumer<'_>) -> String
     show_reg(reg)
 }
 
+pub fn pretty_print_regpair(pair: RegPair, allocs: &mut AllocationConsumer<'_>) -> String {
+    let hi = allocs.next(pair.hi);
+    let lo = allocs.next(pair.lo);
+    if let Some(hi_reg) = hi.to_real_reg() {
+        if let Some(lo_reg) = lo.to_real_reg() {
+            assert!(
+                hi_reg.hw_enc() + 1 == lo_reg.hw_enc(),
+                "Invalid regpair: {} {}",
+                show_reg(hi),
+                show_reg(lo)
+            );
+            return show_reg(hi);
+        }
+    }
+
+    format!("{}/{}", show_reg(hi), show_reg(lo))
+}
+
 pub fn pretty_print_reg_mod(
     rd: Writable<Reg>,
     ri: Reg,
@@ -192,6 +211,48 @@ pub fn pretty_print_reg_mod(
     }
 }
 
+pub fn pretty_print_regpair_mod(
+    rd: WritableRegPair,
+    ri: RegPair,
+    allocs: &mut AllocationConsumer<'_>,
+) -> String {
+    let rd_hi = allocs.next(rd.hi.to_reg());
+    let rd_lo = allocs.next(rd.lo.to_reg());
+    let ri_hi = allocs.next(ri.hi);
+    let ri_lo = allocs.next(ri.lo);
+    if rd_hi == ri_hi {
+        show_reg(rd_hi)
+    } else {
+        format!(
+            "{}/{}<-{}/{}",
+            show_reg(rd_hi),
+            show_reg(rd_lo),
+            show_reg(ri_hi),
+            show_reg(ri_lo)
+        )
+    }
+}
+
+pub fn pretty_print_regpair_mod_lo(
+    rd: WritableRegPair,
+    ri: Reg,
+    allocs: &mut AllocationConsumer<'_>,
+) -> String {
+    let rd_hi = allocs.next(rd.hi.to_reg());
+    let rd_lo = allocs.next(rd.lo.to_reg());
+    let ri = allocs.next(ri);
+    if rd_lo == ri {
+        show_reg(rd_hi)
+    } else {
+        format!(
+            "{}/{}<-_/{}",
+            show_reg(rd_hi),
+            show_reg(rd_lo),
+            show_reg(ri),
+        )
+    }
+}
+
 pub fn pretty_print_fpr(reg: Reg, allocs: &mut AllocationConsumer<'_>) -> (String, Option<String>) {
     let reg = allocs.next(reg);
     (show_reg(reg), maybe_show_fpr(reg))