Auto merge of #124905 - reitermarkus:u32-from-char-opt, r=scottmcm

Allow optimizing `u32::from::<char>`.

Extracted from https://github.com/rust-lang/rust/pull/124307.

This allows optimizing the panicking branch in the `escape_unicode` function, see https://rust.godbolt.org/z/61YhKrhvP.

1 files changed, 35 insertions, 0 deletions

diff --git a/tests/codegen/cast-optimized.rs b/tests/codegen/cast-optimized.rs
new file mode 100644
index 00000000000..313b2b4f0d6
--- /dev/null
+++ b/tests/codegen/cast-optimized.rs
@@ -0,0 +1,35 @@
+//@ compile-flags: -O -Z merge-functions=disabled
+#![crate_type = "lib"]
+
+// This tests that LLVM can optimize based on the niches in the source or
+// destination types for casts.
+
+// CHECK-LABEL: @u32_index
+#[no_mangle]
+pub fn u32_index(c: u32) -> [bool; 22] {
+    let mut array = [false; 22];
+
+    let index = 32 - c.leading_zeros();
+
+    // CHECK: call core::panicking::panic
+    array[index as usize] = true;
+
+    array
+}
+
+// CHECK-LABEL: @char_as_u32_index
+#[no_mangle]
+pub fn char_as_u32_index(c: char) -> [bool; 22] {
+    // CHECK: %[[B:.+]] = icmp ult i32 %c, 1114112
+    // CHECK: call void @llvm.assume(i1 %[[B]])
+    let c = c as u32;
+
+    let mut array = [false; 22];
+
+    let index = 32 - c.leading_zeros();
+
+    // CHECK-NOT: call core::panicking::panic
+    array[index as usize] = true;
+
+    array
+}