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.)

5 files changed, 66 insertions, 39 deletions

diff --git a/compiler/rustc_codegen_gcc/src/builder.rs b/compiler/rustc_codegen_gcc/src/builder.rs
index 1c3d6cc899a..b8e37b60480 100644
--- a/compiler/rustc_codegen_gcc/src/builder.rs
+++ b/compiler/rustc_codegen_gcc/src/builder.rs
@@ -665,6 +665,7 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
         a + b
     }
 
+    // TODO(antoyo): should we also override the `unchecked_` versions?
     fn sub(&mut self, a: RValue<'gcc>, b: RValue<'gcc>) -> RValue<'gcc> {
         self.gcc_sub(a, b)
     }
@@ -832,31 +833,6 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
         set_rvalue_location(self, self.gcc_not(a))
     }
 
-    fn unchecked_sadd(&mut self, a: RValue<'gcc>, b: RValue<'gcc>) -> RValue<'gcc> {
-        set_rvalue_location(self, self.gcc_add(a, b))
-    }
-
-    fn unchecked_uadd(&mut self, a: RValue<'gcc>, b: RValue<'gcc>) -> RValue<'gcc> {
-        set_rvalue_location(self, self.gcc_add(a, b))
-    }
-
-    fn unchecked_ssub(&mut self, a: RValue<'gcc>, b: RValue<'gcc>) -> RValue<'gcc> {
-        set_rvalue_location(self, self.gcc_sub(a, b))
-    }
-
-    fn unchecked_usub(&mut self, a: RValue<'gcc>, b: RValue<'gcc>) -> RValue<'gcc> {
-        // TODO(antoyo): should generate poison value?
-        set_rvalue_location(self, self.gcc_sub(a, b))
-    }
-
-    fn unchecked_smul(&mut self, a: RValue<'gcc>, b: RValue<'gcc>) -> RValue<'gcc> {
-        set_rvalue_location(self, self.gcc_mul(a, b))
-    }
-
-    fn unchecked_umul(&mut self, a: RValue<'gcc>, b: RValue<'gcc>) -> RValue<'gcc> {
-        set_rvalue_location(self, self.gcc_mul(a, b))
-    }
-
     fn fadd_fast(&mut self, lhs: RValue<'gcc>, rhs: RValue<'gcc>) -> RValue<'gcc> {
         // NOTE: it seems like we cannot enable fast-mode for a single operation in GCC.
         set_rvalue_location(self, lhs + rhs)
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>(
diff --git a/compiler/rustc_codegen_ssa/src/size_of_val.rs b/compiler/rustc_codegen_ssa/src/size_of_val.rs
index 71a2f916db5..ac2366340fb 100644
--- a/compiler/rustc_codegen_ssa/src/size_of_val.rs
+++ b/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()),
             )
         }
diff --git a/compiler/rustc_codegen_ssa/src/traits/builder.rs b/compiler/rustc_codegen_ssa/src/traits/builder.rs
index d9bb5ae773e..2c843e2f5e4 100644
--- a/compiler/rustc_codegen_ssa/src/traits/builder.rs
+++ b/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