Add new intrinsic `is_constant` and optimize `pow`

Fix overflow check

Make MIRI choose the path randomly and rename the intrinsic

Add back test

Add miri test and make it operate on `ptr`

Define `llvm.is.constant` for primitives

Update MIRI comment and fix test in stage2

Add const eval test

Clarify that both branches must have the same side effects

guaranteed non guarantee

use immediate type instead

Co-Authored-By: Ralf Jung <post@ralfj.de>

5 files changed, 26 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_llvm/src/context.rs b/compiler/rustc_codegen_llvm/src/context.rs
index 1d1b6e6148d..56fa14e3cee 100644
--- a/compiler/rustc_codegen_llvm/src/context.rs
+++ b/compiler/rustc_codegen_llvm/src/context.rs
@@ -908,6 +908,20 @@ impl<'ll> CodegenCx<'ll, '_> {
         ifn!("llvm.lifetime.start.p0i8", fn(t_i64, ptr) -> void);
         ifn!("llvm.lifetime.end.p0i8", fn(t_i64, ptr) -> void);
 
+        // FIXME: This is an infinitesimally small portion of the types you can
+        // pass to this intrinsic, if we can ever lazily register intrinsics we
+        // should register these when they're used, that way any type can be
+        // passed.
+        ifn!("llvm.is.constant.i1", fn(i1) -> i1);
+        ifn!("llvm.is.constant.i8", fn(t_i8) -> i1);
+        ifn!("llvm.is.constant.i16", fn(t_i16) -> i1);
+        ifn!("llvm.is.constant.i32", fn(t_i32) -> i1);
+        ifn!("llvm.is.constant.i64", fn(t_i64) -> i1);
+        ifn!("llvm.is.constant.i128", fn(t_i128) -> i1);
+        ifn!("llvm.is.constant.isize", fn(t_isize) -> i1);
+        ifn!("llvm.is.constant.f32", fn(t_f32) -> i1);
+        ifn!("llvm.is.constant.f64", fn(t_f64) -> i1);
+
         ifn!("llvm.expect.i1", fn(i1, i1) -> i1);
         ifn!("llvm.eh.typeid.for", fn(ptr) -> t_i32);
         ifn!("llvm.localescape", fn(...) -> void);
diff --git a/compiler/rustc_codegen_llvm/src/intrinsic.rs b/compiler/rustc_codegen_llvm/src/intrinsic.rs
index a0f9d5cf7cd..f1a6f7bd8e6 100644
--- a/compiler/rustc_codegen_llvm/src/intrinsic.rs
+++ b/compiler/rustc_codegen_llvm/src/intrinsic.rs
@@ -119,6 +119,10 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
             sym::likely => {
                 self.call_intrinsic("llvm.expect.i1", &[args[0].immediate(), self.const_bool(true)])
             }
+            sym::is_val_statically_known => self.call_intrinsic(
+                &format!("llvm.is.constant.{:?}", args[0].layout.immediate_llvm_type(self.cx)),
+                &[args[0].immediate()],
+            ),
             sym::unlikely => self
                 .call_intrinsic("llvm.expect.i1", &[args[0].immediate(), self.const_bool(false)]),
             kw::Try => {
diff --git a/compiler/rustc_const_eval/src/const_eval/machine.rs b/compiler/rustc_const_eval/src/const_eval/machine.rs
index 7fb5f10c6ca..9265e2b4252 100644
--- a/compiler/rustc_const_eval/src/const_eval/machine.rs
+++ b/compiler/rustc_const_eval/src/const_eval/machine.rs
@@ -531,6 +531,11 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
                     )?;
                 }
             }
+            // The intrinsic represents whether the value is known to the optimizer (LLVM).
+            // We're not doing any optimizations here, so there is no optimizer that could know the value.
+            // (We know the value here in the machine of course, but this is the runtime of that code,
+            // not the optimization stage.)
+            sym::is_val_statically_known => ecx.write_scalar(Scalar::from_bool(false), dest)?,
             _ => {
                 throw_unsup_format!(
                     "intrinsic `{intrinsic_name}` is not supported at compile-time"
diff --git a/compiler/rustc_hir_analysis/src/check/intrinsic.rs b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
index a5aedeb33ae..40958f7a6e3 100644
--- a/compiler/rustc_hir_analysis/src/check/intrinsic.rs
+++ b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
@@ -453,6 +453,8 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
 
             sym::black_box => (1, vec![param(0)], param(0)),
 
+            sym::is_val_statically_known => (1, vec![param(0)], tcx.types.bool),
+
             sym::const_eval_select => (4, vec![param(0), param(1), param(2)], param(3)),
 
             sym::vtable_size | sym::vtable_align => {
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 8ed1255c010..c09e4dfcf7d 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -907,6 +907,7 @@ symbols! {
         io_stderr,
         io_stdout,
         irrefutable_let_patterns,
+        is_val_statically_known,
         isa_attribute,
         isize,
         issue,