Auto merge of #136575 - scottmcm:nsuw-math, r=nikic

Set both `nuw` and `nsw` in slice size calculation

There's an old note in the code to do this, and now that [LLVM-C has an API for it](https://github.com/llvm/llvm-project/blob/f0b8ff12519270adcfef93410abff76ab073476a/llvm/include/llvm-c/Core.h#L4403-L4408), we might as well.  And it's been there since what looks like LLVM 17 https://github.com/llvm/llvm-project/commit/de9b6aa341d8951625d62ae3dac8670ebb3eb006 so doesn't even need to be conditional.

(There's other places, like `RawVecInner` or `Layout`, that might want to do things like this too, but I'll leave those for a future PR.)

2 files changed, 33 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_llvm/src/builder.rs b/compiler/rustc_codegen_llvm/src/builder.rs
index acae0b444c0..57f09b1fa2f 100644
--- a/compiler/rustc_codegen_llvm/src/builder.rs
+++ b/compiler/rustc_codegen_llvm/src/builder.rs
@@ -421,6 +421,37 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         unchecked_umul(x, y) => LLVMBuildNUWMul,
     }
 
+    fn unchecked_suadd(&mut self, a: &'ll Value, b: &'ll Value) -> &'ll Value {
+        unsafe {
+            let add = llvm::LLVMBuildAdd(self.llbuilder, a, b, UNNAMED);
+            if llvm::LLVMIsAInstruction(add).is_some() {
+                llvm::LLVMSetNUW(add, True);
+                llvm::LLVMSetNSW(add, True);
+            }
+            add
+        }
+    }
+    fn unchecked_susub(&mut self, a: &'ll Value, b: &'ll Value) -> &'ll Value {
+        unsafe {
+            let sub = llvm::LLVMBuildSub(self.llbuilder, a, b, UNNAMED);
+            if llvm::LLVMIsAInstruction(sub).is_some() {
+                llvm::LLVMSetNUW(sub, True);
+                llvm::LLVMSetNSW(sub, True);
+            }
+            sub
+        }
+    }
+    fn unchecked_sumul(&mut self, a: &'ll Value, b: &'ll Value) -> &'ll Value {
+        unsafe {
+            let mul = llvm::LLVMBuildMul(self.llbuilder, a, b, UNNAMED);
+            if llvm::LLVMIsAInstruction(mul).is_some() {
+                llvm::LLVMSetNUW(mul, True);
+                llvm::LLVMSetNSW(mul, True);
+            }
+            mul
+        }
+    }
+
     fn or_disjoint(&mut self, a: &'ll Value, b: &'ll Value) -> &'ll Value {
         unsafe {
             let or = llvm::LLVMBuildOr(self.llbuilder, a, b, UNNAMED);
diff --git a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
index 618fdb09d4d..3b0187b9d37 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
@@ -1430,6 +1430,8 @@ unsafe extern "C" {
 
     // Extra flags on arithmetic
     pub(crate) fn LLVMSetIsDisjoint(Instr: &Value, IsDisjoint: Bool);
+    pub(crate) fn LLVMSetNUW(ArithInst: &Value, HasNUW: Bool);
+    pub(crate) fn LLVMSetNSW(ArithInst: &Value, HasNSW: Bool);
 
     // Memory
     pub(crate) fn LLVMBuildAlloca<'a>(