Emit fewer assumes now that we have range metadata on parameters

scottmcm · scottmcm · commit 959fd5f1c759 · 2025-01-16T20:15:53.000-08:00
We still need the `assume` for the *target* type's range, but we no longer need it for the *source* type's range.

Admittedly there's one test not properly handled by LLVM today, but it's synthetic, so I'd still be fine doing this and just updating the test once LLVM fixes the bug.  All the other optimization tests still pass.

Hopefully this means less crud for LLVM to churn through in `opt` builds...
diff --git a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs b/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
@@ -257,13 +257,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 if let OperandValueKind::Immediate(to_scalar) = cast_kind
                     && from_scalar.size(self.cx) == to_scalar.size(self.cx)
                 {
-                    let from_backend_ty = bx.backend_type(operand.layout);
                     let to_backend_ty = bx.backend_type(cast);
                     Some(OperandValue::Immediate(self.transmute_immediate(
                         bx,
                         imm,
                         from_scalar,
-                        from_backend_ty,
                         to_scalar,
                         to_backend_ty,
                     )))
@@ -279,13 +277,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     && in_a.size(self.cx) == out_a.size(self.cx)
                     && in_b.size(self.cx) == out_b.size(self.cx)
                 {
-                    let in_a_ibty = bx.scalar_pair_element_backend_type(operand.layout, 0, false);
-                    let in_b_ibty = bx.scalar_pair_element_backend_type(operand.layout, 1, false);
                     let out_a_ibty = bx.scalar_pair_element_backend_type(cast, 0, false);
                     let out_b_ibty = bx.scalar_pair_element_backend_type(cast, 1, false);
                     Some(OperandValue::Pair(
-                        self.transmute_immediate(bx, imm_a, in_a, in_a_ibty, out_a, out_a_ibty),
-                        self.transmute_immediate(bx, imm_b, in_b, in_b_ibty, out_b, out_b_ibty),
+                        self.transmute_immediate(bx, imm_a, in_a, out_a, out_a_ibty),
+                        self.transmute_immediate(bx, imm_b, in_b, out_b, out_b_ibty),
                     ))
                 } else {
                     None
@@ -356,7 +352,6 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         bx: &mut Bx,
         mut imm: Bx::Value,
         from_scalar: abi::Scalar,
-        from_backend_ty: Bx::Type,
         to_scalar: abi::Scalar,
         to_backend_ty: Bx::Type,
     ) -> Bx::Value {
@@ -365,11 +360,13 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         use abi::Primitive::*;
         imm = bx.from_immediate(imm);
 
-        // When scalars are passed by value, there's no metadata recording their
-        // valid ranges. For example, `char`s are passed as just `i32`, with no
-        // way for LLVM to know that they're 0x10FFFF at most. Thus we assume
-        // the range of the input value too, not just the output range.
-        self.assume_scalar_range(bx, imm, from_scalar, from_backend_ty);
+        // We used to `assume` the `from_scalar` here too, but that's no longer needed
+        // because if we have a scalar, we must already know its range. Either
+        // 1) It's a parameter with `range` parameter metadata,
+        // 2) It's something we `load`ed with `!range` metadata, or
+        // 3) It's something we transmuted and already `assume`d the range.
+        // And thus in all those cases another `assume` is just wasteful.
+        // (Case 1 didn't used to be covered, and thus the `assume` was needed.)
 
         imm = match (from_scalar.primitive(), to_scalar.primitive()) {
             (Int(..) | Float(_), Int(..) | Float(_)) => bx.bitcast(imm, to_backend_ty),
@@ -389,7 +386,14 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 bx.bitcast(int_imm, to_backend_ty)
             }
         };
+
+        // This `assume` remains important for cases like
+        //    transmute::<u32, NonZeroU32>(x) == 0
+        // since it's never passed to something with parameter metadata (especially
+        // after MIR inlining) so the only way to tell the backend about the
+        // constraint that the `transmute` introduced is to `assume` it.
         self.assume_scalar_range(bx, imm, to_scalar, to_backend_ty);
+
         imm = bx.to_immediate_scalar(imm, to_scalar);
         imm
     }
diff --git a/tests/codegen/intrinsics/transmute-niched.rs b/tests/codegen/intrinsics/transmute-niched.rs
@@ -17,26 +17,25 @@ pub enum SmallEnum {
 // CHECK-LABEL: @check_to_enum(
 #[no_mangle]
 pub unsafe fn check_to_enum(x: i8) -> SmallEnum {
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
     // OPT: %0 = icmp uge i8 %x, 10
     // OPT: call void @llvm.assume(i1 %0)
     // OPT: %1 = icmp ule i8 %x, 12
     // OPT: call void @llvm.assume(i1 %1)
-    // DBG-NOT: icmp
-    // DBG-NOT: assume
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
     // CHECK: ret i8 %x
 
     transmute(x)
 }
 
 // CHECK-LABEL: @check_from_enum(
+// OPT-SAME: range(i8 10, 13){{.+}}%x
 #[no_mangle]
 pub unsafe fn check_from_enum(x: SmallEnum) -> i8 {
-    // OPT: %0 = icmp uge i8 %x, 10
-    // OPT: call void @llvm.assume(i1 %0)
-    // OPT: %1 = icmp ule i8 %x, 12
-    // OPT: call void @llvm.assume(i1 %1)
-    // DBG-NOT: icmp
-    // DBG-NOT: assume
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
     // CHECK: ret i8 %x
 
     transmute(x)
@@ -45,26 +44,25 @@ pub unsafe fn check_from_enum(x: SmallEnum) -> i8 {
 // CHECK-LABEL: @check_to_ordering(
 #[no_mangle]
 pub unsafe fn check_to_ordering(x: u8) -> std::cmp::Ordering {
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
     // OPT: %0 = icmp uge i8 %x, -1
     // OPT: %1 = icmp ule i8 %x, 1
     // OPT: %2 = or i1 %0, %1
     // OPT: call void @llvm.assume(i1 %2)
-    // DBG-NOT: icmp
-    // DBG-NOT: assume
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
     // CHECK: ret i8 %x
 
     transmute(x)
 }
 
 // CHECK-LABEL: @check_from_ordering(
+// OPT-SAME: range(i8 -1, 2){{.+}}%x
 #[no_mangle]
 pub unsafe fn check_from_ordering(x: std::cmp::Ordering) -> u8 {
-    // OPT: %0 = icmp uge i8 %x, -1
-    // OPT: %1 = icmp ule i8 %x, 1
-    // OPT: %2 = or i1 %0, %1
-    // OPT: call void @llvm.assume(i1 %2)
-    // DBG-NOT: icmp
-    // DBG-NOT: assume
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
     // CHECK: ret i8 %x
 
     transmute(x)
@@ -96,50 +94,50 @@ pub enum Minus100ToPlus100 {
 }
 
 // CHECK-LABEL: @check_enum_from_char(
+// OPT-SAME: range(i32 0, 1114112){{.+}}%x
 #[no_mangle]
 pub unsafe fn check_enum_from_char(x: char) -> Minus100ToPlus100 {
-    // OPT: %0 = icmp ule i32 %x, 1114111
-    // OPT: call void @llvm.assume(i1 %0)
-    // OPT: %1 = icmp uge i32 %x, -100
-    // OPT: %2 = icmp ule i32 %x, 100
-    // OPT: %3 = or i1 %1, %2
-    // OPT: call void @llvm.assume(i1 %3)
-    // DBG-NOT: icmp
-    // DBG-NOT: assume
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
+    // OPT: %0 = icmp uge i32 %x, -100
+    // OPT: %1 = icmp ule i32 %x, 100
+    // OPT: %2 = or i1 %0, %1
+    // OPT: call void @llvm.assume(i1 %2)
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
     // CHECK: ret i32 %x
 
     transmute(x)
 }
 
 // CHECK-LABEL: @check_enum_to_char(
+// OPT-SAME: range(i32 -100, 101){{.+}}%x
 #[no_mangle]
 pub unsafe fn check_enum_to_char(x: Minus100ToPlus100) -> char {
-    // OPT: %0 = icmp uge i32 %x, -100
-    // OPT: %1 = icmp ule i32 %x, 100
-    // OPT: %2 = or i1 %0, %1
-    // OPT: call void @llvm.assume(i1 %2)
-    // OPT: %3 = icmp ule i32 %x, 1114111
-    // OPT: call void @llvm.assume(i1 %3)
-    // DBG-NOT: icmp
-    // DBG-NOT: assume
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
+    // OPT: %0 = icmp ule i32 %x, 1114111
+    // OPT: call void @llvm.assume(i1 %0)
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
     // CHECK: ret i32 %x
 
     transmute(x)
 }
 
 // CHECK-LABEL: @check_swap_pair(
+// OPT-SAME: range(i32 0, 1114112){{.+}}%x.0
+// OPT-SAME: range(i32 1, 0){{.+}}%x.1
 #[no_mangle]
 pub unsafe fn check_swap_pair(x: (char, NonZero<u32>)) -> (NonZero<u32>, char) {
-    // OPT: %0 = icmp ule i32 %x.0, 1114111
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
+    // OPT: %0 = icmp uge i32 %x.0, 1
     // OPT: call void @llvm.assume(i1 %0)
-    // OPT: %1 = icmp uge i32 %x.0, 1
+    // OPT: %1 = icmp ule i32 %x.1, 1114111
     // OPT: call void @llvm.assume(i1 %1)
-    // OPT: %2 = icmp uge i32 %x.1, 1
-    // OPT: call void @llvm.assume(i1 %2)
-    // OPT: %3 = icmp ule i32 %x.1, 1114111
-    // OPT: call void @llvm.assume(i1 %3)
-    // DBG-NOT: icmp
-    // DBG-NOT: assume
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
     // CHECK: %[[P1:.+]] = insertvalue { i32, i32 } poison, i32 %x.0, 0
     // CHECK: %[[P2:.+]] = insertvalue { i32, i32 } %[[P1]], i32 %x.1, 1
     // CHECK: ret { i32, i32 } %[[P2]]
@@ -148,34 +146,34 @@ pub unsafe fn check_swap_pair(x: (char, NonZero<u32>)) -> (NonZero<u32>, char) {
 }
 
 // CHECK-LABEL: @check_bool_from_ordering(
+// OPT-SAME: range(i8 -1, 2){{.+}}%x
 #[no_mangle]
 pub unsafe fn check_bool_from_ordering(x: std::cmp::Ordering) -> bool {
-    // OPT: %0 = icmp uge i8 %x, -1
-    // OPT: %1 = icmp ule i8 %x, 1
-    // OPT: %2 = or i1 %0, %1
-    // OPT: call void @llvm.assume(i1 %2)
-    // OPT: %3 = icmp ule i8 %x, 1
-    // OPT: call void @llvm.assume(i1 %3)
-    // DBG-NOT: icmp
-    // DBG-NOT: assume
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
+    // OPT: %0 = icmp ule i8 %x, 1
+    // OPT: call void @llvm.assume(i1 %0)
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
     // CHECK: %[[R:.+]] = trunc i8 %x to i1
     // CHECK: ret i1 %[[R]]
 
     transmute(x)
 }
 
 // CHECK-LABEL: @check_bool_to_ordering(
+// OPT-SAME: i1 {{.+}} %x
 #[no_mangle]
 pub unsafe fn check_bool_to_ordering(x: bool) -> std::cmp::Ordering {
     // CHECK: %_0 = zext i1 %x to i8
-    // OPT: %0 = icmp ule i8 %_0, 1
-    // OPT: call void @llvm.assume(i1 %0)
-    // OPT: %1 = icmp uge i8 %_0, -1
-    // OPT: %2 = icmp ule i8 %_0, 1
-    // OPT: %3 = or i1 %1, %2
-    // OPT: call void @llvm.assume(i1 %3)
-    // DBG-NOT: icmp
-    // DBG-NOT: assume
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
+    // OPT: %0 = icmp uge i8 %_0, -1
+    // OPT: %1 = icmp ule i8 %_0, 1
+    // OPT: %2 = or i1 %0, %1
+    // OPT: call void @llvm.assume(i1 %2)
+    // CHECK-NOT: icmp
+    // CHECK-NOT: assume
     // CHECK: ret i8 %_0
 
     transmute(x)
diff --git a/tests/codegen/transmute-optimized.rs b/tests/codegen/transmute-optimized.rs
@@ -90,9 +90,18 @@ pub enum OneTwoThree {
 }
 
 // CHECK-LABEL: i8 @ordering_transmute_onetwothree(i8
+// CHECK-SAME: range(i8 -1, 2){{.+}}%x
 #[no_mangle]
 pub unsafe fn ordering_transmute_onetwothree(x: std::cmp::Ordering) -> OneTwoThree {
-    // CHECK: ret i8 1
+    // FIXME: this *should* just be `ret i8 1`, but that's not happening today.
+    // cc <https://github.com/llvm/llvm-project/issues/123278>
+
+    // CHECK: %[[TEMP1:.+]] = icmp ne i8 %x, 0
+    // CHECK: tail call void @llvm.assume(i1 %[[TEMP1]])
+    // CHECK: %[[TEMP2:.+]] = icmp ult i8 %x, 4
+    // CHECK: tail call void @llvm.assume(i1 %[[TEMP2]])
+
+    // CHECK: ret i8 %x
     std::mem::transmute(x)
 }
 
@@ -110,3 +119,11 @@ pub fn char_is_negative(c: char) -> bool {
     let x: i32 = unsafe { std::mem::transmute(c) };
     x < 0
 }
+
+// CHECK-LABEL: i1 @transmute_to_char_is_negative(i32
+#[no_mangle]
+pub fn transmute_to_char_is_negative(x: i32) -> bool {
+    // CHECK: ret i1 false
+    let _c: char = unsafe { std::mem::transmute(x) };
+    x < 0
+}
diff --git a/tests/codegen/vec-in-place.rs b/tests/codegen/vec-in-place.rs
@@ -41,9 +41,6 @@ pub fn vec_iterator_cast_primitive(vec: Vec<i8>) -> Vec<u8> {
     // CHECK: call{{.+}}void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: loop
     // CHECK-NOT: call
-    // CHECK: call{{.+}}void @llvm.assume(i1 %{{.+}})
-    // CHECK-NOT: loop
-    // CHECK-NOT: call
     vec.into_iter().map(|e| e as u8).collect()
 }
 
@@ -55,9 +52,6 @@ pub fn vec_iterator_cast_wrapper(vec: Vec<u8>) -> Vec<Wrapper<u8>> {
     // CHECK: call{{.+}}void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: loop
     // CHECK-NOT: call
-    // CHECK: call{{.+}}void @llvm.assume(i1 %{{.+}})
-    // CHECK-NOT: loop
-    // CHECK-NOT: call
     vec.into_iter().map(|e| Wrapper(e)).collect()
 }
 
@@ -86,9 +80,6 @@ pub fn vec_iterator_cast_unwrap(vec: Vec<Wrapper<u8>>) -> Vec<u8> {
     // CHECK: call{{.+}}void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: loop
     // CHECK-NOT: call
-    // CHECK: call{{.+}}void @llvm.assume(i1 %{{.+}})
-    // CHECK-NOT: loop
-    // CHECK-NOT: call
     vec.into_iter().map(|e| e.0).collect()
 }
 
@@ -100,9 +91,6 @@ pub fn vec_iterator_cast_aggregate(vec: Vec<[u64; 4]>) -> Vec<Foo> {
     // CHECK: call{{.+}}void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: loop
     // CHECK-NOT: call
-    // CHECK: call{{.+}}void @llvm.assume(i1 %{{.+}})
-    // CHECK-NOT: loop
-    // CHECK-NOT: call
     vec.into_iter().map(|e| unsafe { std::mem::transmute(e) }).collect()
 }
 
@@ -114,9 +102,6 @@ pub fn vec_iterator_cast_deaggregate_tra(vec: Vec<Bar>) -> Vec<[u64; 4]> {
     // CHECK: call{{.+}}void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: loop
     // CHECK-NOT: call
-    // CHECK: call{{.+}}void @llvm.assume(i1 %{{.+}})
-    // CHECK-NOT: loop
-    // CHECK-NOT: call
 
     // Safety: For the purpose of this test we assume that Bar layout matches [u64; 4].
     // This currently is not guaranteed for repr(Rust) types, but it happens to work here and
@@ -133,9 +118,6 @@ pub fn vec_iterator_cast_deaggregate_fold(vec: Vec<Baz>) -> Vec<[u64; 4]> {
     // CHECK: call{{.+}}void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: loop
     // CHECK-NOT: call
-    // CHECK: call{{.+}}void @llvm.assume(i1 %{{.+}})
-    // CHECK-NOT: loop
-    // CHECK-NOT: call
 
     // Safety: For the purpose of this test we assume that Bar layout matches [u64; 4].
     // This currently is not guaranteed for repr(Rust) types, but it happens to work here and
@@ -156,12 +138,6 @@ pub fn vec_iterator_cast_unwrap_drop(vec: Vec<Wrapper<String>>) -> Vec<String> {
     // CHECK-NOT: call
     // CHECK-NOT: %{{.*}} = mul
     // CHECK-NOT: %{{.*}} = udiv
-    // CHECK: call
-    // CHECK-SAME: void @llvm.assume(i1 %{{.+}})
-    // CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
-    // CHECK-NOT: call
-    // CHECK-NOT: %{{.*}} = mul
-    // CHECK-NOT: %{{.*}} = udiv
 
     vec.into_iter().map(|Wrapper(e)| e).collect()
 }
@@ -178,12 +154,6 @@ pub fn vec_iterator_cast_wrap_drop(vec: Vec<String>) -> Vec<Wrapper<String>> {
     // CHECK-NOT: call
     // CHECK-NOT: %{{.*}} = mul
     // CHECK-NOT: %{{.*}} = udiv
-    // CHECK: call
-    // CHECK-SAME: void @llvm.assume(i1 %{{.+}})
-    // CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
-    // CHECK-NOT: call
-    // CHECK-NOT: %{{.*}} = mul
-    // CHECK-NOT: %{{.*}} = udiv
     // CHECK: ret void
 
     vec.into_iter().map(Wrapper).collect()
