Rollup merge of #126849 - workingjubilee:correctly-classify-arm-low-dregs, r=Amanieu

Fix 32-bit Arm reg classes by hierarchically sorting them

We were rejecting legal `asm!` because we were asking for the "greatest" feature that includes a register class, instead of the "least" feature that includes a register class. This was only revealed on certain 32-bit Arm targets because not all have the same register limitations.

This is a somewhat hacky solution, but other solutions would require potentially rearchitecting how the internals of parsing or rejecting register classes work for all targets.

Fixes #126797

r​? ``@Amanieu``

compiler/rustc_target/src/asm/arm.rs

@@ -148,22 +148,22 @@ fn reserved_r9(
         r11: reg = ["r11", "fp"] % frame_pointer_r11,
         r12: reg = ["r12", "ip"] % not_thumb1,
         r14: reg = ["r14", "lr"] % not_thumb1,
-        s0: sreg, sreg_low16 = ["s0"],
-        s1: sreg, sreg_low16 = ["s1"],
-        s2: sreg, sreg_low16 = ["s2"],
-        s3: sreg, sreg_low16 = ["s3"],
-        s4: sreg, sreg_low16 = ["s4"],
-        s5: sreg, sreg_low16 = ["s5"],
-        s6: sreg, sreg_low16 = ["s6"],
-        s7: sreg, sreg_low16 = ["s7"],
-        s8: sreg, sreg_low16 = ["s8"],
-        s9: sreg, sreg_low16 = ["s9"],
-        s10: sreg, sreg_low16 = ["s10"],
-        s11: sreg, sreg_low16 = ["s11"],
-        s12: sreg, sreg_low16 = ["s12"],
-        s13: sreg, sreg_low16 = ["s13"],
-        s14: sreg, sreg_low16 = ["s14"],
-        s15: sreg, sreg_low16 = ["s15"],
+        s0: sreg_low16, sreg = ["s0"],
+        s1: sreg_low16, sreg = ["s1"],
+        s2: sreg_low16, sreg = ["s2"],
+        s3: sreg_low16, sreg = ["s3"],
+        s4: sreg_low16, sreg = ["s4"],
+        s5: sreg_low16, sreg = ["s5"],
+        s6: sreg_low16, sreg = ["s6"],
+        s7: sreg_low16, sreg = ["s7"],
+        s8: sreg_low16, sreg = ["s8"],
+        s9: sreg_low16, sreg = ["s9"],
+        s10: sreg_low16, sreg = ["s10"],
+        s11: sreg_low16, sreg = ["s11"],
+        s12: sreg_low16, sreg = ["s12"],
+        s13: sreg_low16, sreg = ["s13"],
+        s14: sreg_low16, sreg = ["s14"],
+        s15: sreg_low16, sreg = ["s15"],
         s16: sreg = ["s16"],
         s17: sreg = ["s17"],
         s18: sreg = ["s18"],
@@ -180,22 +180,22 @@ fn reserved_r9(
         s29: sreg = ["s29"],
         s30: sreg = ["s30"],
         s31: sreg = ["s31"],
-        d0: dreg, dreg_low16, dreg_low8 = ["d0"],
-        d1: dreg, dreg_low16, dreg_low8 = ["d1"],
-        d2: dreg, dreg_low16, dreg_low8 = ["d2"],
-        d3: dreg, dreg_low16, dreg_low8 = ["d3"],
-        d4: dreg, dreg_low16, dreg_low8 = ["d4"],
-        d5: dreg, dreg_low16, dreg_low8 = ["d5"],
-        d6: dreg, dreg_low16, dreg_low8 = ["d6"],
-        d7: dreg, dreg_low16, dreg_low8 = ["d7"],
-        d8: dreg, dreg_low16 = ["d8"],
-        d9: dreg, dreg_low16 = ["d9"],
-        d10: dreg, dreg_low16 = ["d10"],
-        d11: dreg, dreg_low16 = ["d11"],
-        d12: dreg, dreg_low16 = ["d12"],
-        d13: dreg, dreg_low16 = ["d13"],
-        d14: dreg, dreg_low16 = ["d14"],
-        d15: dreg, dreg_low16 = ["d15"],
+        d0: dreg_low8, dreg_low16, dreg = ["d0"],
+        d1: dreg_low8, dreg_low16, dreg = ["d1"],
+        d2: dreg_low8, dreg_low16, dreg = ["d2"],
+        d3: dreg_low8, dreg_low16, dreg = ["d3"],
+        d4: dreg_low8, dreg_low16, dreg = ["d4"],
+        d5: dreg_low8, dreg_low16, dreg = ["d5"],
+        d6: dreg_low8, dreg_low16, dreg = ["d6"],
+        d7: dreg_low8, dreg_low16, dreg = ["d7"],
+        d8: dreg_low16, dreg = ["d8"],
+        d9: dreg_low16, dreg = ["d9"],
+        d10: dreg_low16, dreg = ["d10"],
+        d11: dreg_low16, dreg = ["d11"],
+        d12: dreg_low16, dreg = ["d12"],
+        d13: dreg_low16, dreg = ["d13"],
+        d14: dreg_low16, dreg = ["d14"],
+        d15: dreg_low16, dreg = ["d15"],
         d16: dreg = ["d16"],
         d17: dreg = ["d17"],
         d18: dreg = ["d18"],
@@ -212,14 +212,14 @@ fn reserved_r9(
         d29: dreg = ["d29"],
         d30: dreg = ["d30"],
         d31: dreg = ["d31"],
-        q0: qreg, qreg_low8, qreg_low4 = ["q0"],
-        q1: qreg, qreg_low8, qreg_low4 = ["q1"],
-        q2: qreg, qreg_low8, qreg_low4 = ["q2"],
-        q3: qreg, qreg_low8, qreg_low4 = ["q3"],
-        q4: qreg, qreg_low8 = ["q4"],
-        q5: qreg, qreg_low8 = ["q5"],
-        q6: qreg, qreg_low8 = ["q6"],
-        q7: qreg, qreg_low8 = ["q7"],
+        q0: qreg_low4, qreg_low8, qreg = ["q0"],
+        q1: qreg_low4, qreg_low8, qreg = ["q1"],
+        q2: qreg_low4, qreg_low8, qreg = ["q2"],
+        q3: qreg_low4, qreg_low8, qreg = ["q3"],
+        q4: qreg_low8, qreg = ["q4"],
+        q5: qreg_low8, qreg = ["q5"],
+        q6: qreg_low8, qreg = ["q6"],
+        q7: qreg_low8, qreg = ["q7"],
         q8: qreg = ["q8"],
         q9: qreg = ["q9"],
         q10: qreg = ["q10"],

tests/ui/asm/arm-low-dreg.rs

@@ -0,0 +1,33 @@
+//@ build-pass
+//@ compile-flags: --target=armv7-unknown-linux-gnueabihf
+//@ needs-llvm-components: arm
+#![feature(no_core, rustc_attrs, decl_macro, lang_items)]
+#![crate_type = "rlib"]
+#![no_std]
+#![no_core]
+
+// We accidentally classified "d0"..="d15" as dregs, even though they are in dreg_low16,
+// and thus didn't compile them on platforms with only 16 dregs.
+// Highlighted in https://github.com/rust-lang/rust/issues/126797
+
+#[lang = "sized"]
+trait Sized {}
+
+#[lang = "copy"]
+trait Copy {}
+
+impl Copy for f64 {}
+
+#[rustc_builtin_macro]
+pub macro asm("assembly template", $(operands,)* $(options($(option),*))?) {
+    /* compiler built-in */
+}
+
+
+fn f(x: f64) -> f64 {
+    let out: f64;
+    unsafe {
+        asm!("vmov.f64 d1, d0", out("d1") out, in("d0") x);
+    }
+    out
+}