Rollup merge of rust-lang#136217 - taiki-e:csky-asm-flags, r=Amanieu

Mark condition/carry bit as clobbered in C-SKY inline assembly

C-SKY's compare and some arithmetic/logical instructions modify condition/carry bit (C) in PSR, but there is currently no way to mark it as clobbered in `asm!`.

This PR marks it as clobbered except when [`options(preserves_flags)`](https://doc.rust-lang.org/reference/inline-assembly.html#r-asm.options.supported-options.preserves_flags) is used.

Refs:
- Section 1.3 "Programming model" and Section 1.3.5 "Condition/carry bit" in CSKY Architecture user_guide:
  https://github.com/c-sky/csky-doc/blob/9f7121f7d40970ba5cc0f15716da033db2bb9d07/CSKY%20Architecture%20user_guide.pdf

  > Under user mode, condition/carry bit (C) is located in the lowest bit of PSR, and it can be
accessed and changed by common user instructions. It is the only data bit that can be visited
under user mode in PSR.

  > Condition or carry bit represents the result after one operation. Condition/carry bit can be
clearly set according to the results of compare instructions or unclearly set as some
high-precision arithmetic or logical instructions. In addition, special instructions such as
DEC[GT,LT,NE] and XTRB[0-3] will influence the value of condition/carry bit.

- Register definition in LLVM:
  https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/CSKY/CSKYRegisterInfo.td#L88

cc `@Dirreke` ([target maintainer](https://github.com/rust-lang/rust/blob/aa6f5ab18e67cb815f73e0d53d217bc54b0da924/src/doc/rustc/src/platform-support/csky-unknown-linux-gnuabiv2.md#target-maintainers))

r? `@Amanieu`

`@rustbot` label +O-csky +A-inline-assembly

Author: GuillaumeGomez

compiler/rustc_codegen_llvm/src/asm.rs

@@ -286,7 +286,9 @@ impl<'ll, 'tcx> AsmBuilderMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                 InlineAsmArch::M68k => {
                     constraints.push("~{ccr}".to_string());
                 }
-                InlineAsmArch::CSKY => {}
+                InlineAsmArch::CSKY => {
+                    constraints.push("~{psr}".to_string());
+                }
             }
         }
         if !options.contains(InlineAsmOptions::NOMEM) {

src/doc/unstable-book/src/language-features/asm-experimental-arch.md

@@ -199,3 +199,5 @@ These flags registers must be restored upon exiting the asm block if the `preser
 - SPARC
   - Integer condition codes (`icc` and `xcc`)
   - Floating-point condition codes (`fcc[0-3]`)
+- CSKY
+  - Condition/carry bit (C) in `PSR`.

tests/codegen/asm/csky-clobbers.rs

@@ -0,0 +1,24 @@
+//@ add-core-stubs
+//@ compile-flags: --target csky-unknown-linux-gnuabiv2
+//@ needs-llvm-components: csky
+
+#![crate_type = "rlib"]
+#![feature(no_core, asm_experimental_arch)]
+#![no_core]
+
+extern crate minicore;
+use minicore::*;
+
+// CHECK-LABEL: @flags_clobber
+// CHECK: call void asm sideeffect "", "~{psr}"()
+#[no_mangle]
+pub unsafe fn flags_clobber() {
+    asm!("", options(nostack, nomem));
+}
+
+// CHECK-LABEL: @no_clobber
+// CHECK: call void asm sideeffect "", ""()
+#[no_mangle]
+pub unsafe fn no_clobber() {
+    asm!("", options(nostack, nomem, preserves_flags));
+}