Rollup merge of #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

3 files changed, 29 insertions, 1 deletions

diff --git a/compiler/rustc_codegen_llvm/src/asm.rs b/compiler/rustc_codegen_llvm/src/asm.rs
index 3722d4350a2..be5673eddf9 100644
--- a/compiler/rustc_codegen_llvm/src/asm.rs
+++ b/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) {
diff --git a/src/doc/unstable-book/src/language-features/asm-experimental-arch.md b/src/doc/unstable-book/src/language-features/asm-experimental-arch.md
index c2f4170d7d2..d9566c9f55c 100644
--- a/src/doc/unstable-book/src/language-features/asm-experimental-arch.md
+++ b/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`.
diff --git a/tests/codegen/asm/csky-clobbers.rs b/tests/codegen/asm/csky-clobbers.rs
new file mode 100644
index 00000000000..4986d0fe56d
--- /dev/null
+++ b/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));
+}