add optional soft-float feature for s390x target

to use soft-float one need to also set the `s390x-softfloat` flag in the .rustc_abi target field.
This will automatically enable the +soft-float feature. On normal compilation with the default
ABI soft-float is still marked incompatible. So the ABI cannot be be broken by accident.

Author: fneddy

compiler/rustc_target/src/spec/mod.rs

@@ -1007,6 +1007,8 @@ crate::target_spec_enum! {
         X86Sse2 = "x86-sse2",
         /// On x86-32/64 only: do not use any FPU or SIMD registers for the ABI.
         X86Softfloat = "x86-softfloat",
+        // On S390x only: do not use any FPU or Vector registers for the ABI.
+        S390xSoftFloat = "s390x-softfloat",
     }
 
     parse_error_type = "rustc abi";
@@ -3198,6 +3200,11 @@ impl Target {
                     Arch::X86 | Arch::X86_64,
                     "`x86-softfloat` ABI is only valid for x86 targets"
                 ),
+                RustcAbi::S390xSoftFloat => check_matches!(
+                    self.arch,
+                    Arch::S390x,
+                    "`s390x-softfloat` ABI is only valid for s390x targets"
+                ),
             }
         }
 

compiler/rustc_target/src/target_features.rs

@@ -1098,6 +1098,7 @@ impl Target {
                         // LLVM handles the rest.
                         FeatureConstraints { required: &["soft-float"], incompatible: &[] }
                     }
+                    Some(r) => panic!("invalid Rust ABI for x86: {r:?}"),
                 }
             }
             Arch::X86_64 => {
@@ -1211,11 +1212,26 @@ impl Target {
                 }
             }
             Arch::S390x => {
-                // We don't currently support a softfloat target on this architecture.
-                // As usual, we have to reject swapping the `soft-float` target feature.
+                // Same as x86, We use our own ABI indicator here;
+                // LLVM does not have anything native and will switch ABI based
+                // on the soft-float target feature.
+                // Every case should require or forbid `soft-float`!
                 // The "vector" target feature does not affect the ABI for floats
                 // because the vector and float registers overlap.
-                FeatureConstraints { required: &[], incompatible: &["soft-float"] }
+                match self.rustc_abi {
+                    None => {
+                        // Default hardfloat ABI.
+                        FeatureConstraints { required: &[], incompatible: &["soft-float"] }
+                    }
+                    Some(RustcAbi::S390xSoftFloat) => {
+                        // Softfloat ABI, requires corresponding target feature.
+                        // llvm will switch to soft-float ABI just based on this feature.
+                        FeatureConstraints { required: &["soft-float"], incompatible: &[] }
+                    }
+                    Some(r) => {
+                        panic!("invalid Rust ABI for s390x: {r:?}");
+                    }
+                }
             }
             _ => NOTHING,
         }