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](f0b8ff1251/llvm/include/llvm-c/Core.h (L4403-L4408)), we might as well.  And it's been there since what looks like LLVM 17 de9b6aa341 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.)

compiler/rustc_codegen_ssa/src/size_of_val.rs

@@ -45,14 +45,9 @@ pub fn size_and_align_of_dst<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
             // The info in this case is the length of the str, so the size is that
             // times the unit size.
             (
-                // All slice sizes must fit into `isize`, so this multiplication cannot (signed)
-                // wrap.
-                // NOTE: ideally, we want the effects of both `unchecked_smul` and `unchecked_umul`
-                // (resulting in `mul nsw nuw` in LLVM IR), since we know that the multiplication
-                // cannot signed wrap, and that both operands are non-negative. But at the time of
-                // writing, the `LLVM-C` binding can't do this, and it doesn't seem to enable any
-                // further optimizations.
-                bx.unchecked_smul(info.unwrap(), bx.const_usize(unit.size.bytes())),
+                // All slice sizes must fit into `isize`, so this multiplication cannot
+                // wrap -- neither signed nor unsigned.
+                bx.unchecked_sumul(info.unwrap(), bx.const_usize(unit.size.bytes())),
                 bx.const_usize(unit.align.abi.bytes()),
             )
         }

compiler/rustc_codegen_ssa/src/traits/builder.rs

@@ -159,12 +159,35 @@ pub trait BuilderMethods<'a, 'tcx>:
     /// must be interpreted as unsigned and can be assumed to be less than the size of the left
     /// operand.
     fn ashr(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
-    fn unchecked_sadd(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
-    fn unchecked_uadd(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
-    fn unchecked_ssub(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
-    fn unchecked_usub(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
-    fn unchecked_smul(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
-    fn unchecked_umul(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    fn unchecked_sadd(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value {
+        self.add(lhs, rhs)
+    }
+    fn unchecked_uadd(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value {
+        self.add(lhs, rhs)
+    }
+    fn unchecked_suadd(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value {
+        self.unchecked_sadd(lhs, rhs)
+    }
+    fn unchecked_ssub(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value {
+        self.sub(lhs, rhs)
+    }
+    fn unchecked_usub(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value {
+        self.sub(lhs, rhs)
+    }
+    fn unchecked_susub(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value {
+        self.unchecked_ssub(lhs, rhs)
+    }
+    fn unchecked_smul(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value {
+        self.mul(lhs, rhs)
+    }
+    fn unchecked_umul(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value {
+        self.mul(lhs, rhs)
+    }
+    fn unchecked_sumul(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value {
+        // Which to default to is a fairly arbitrary choice,
+        // but this is what slice layout was using before.
+        self.unchecked_smul(lhs, rhs)
+    }
     fn and(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn or(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     /// Defaults to [`Self::or`], but guarantees `(lhs & rhs) == 0` so some backends