Add support for f128 integer exponentiation

Create the symbol `__powitf2`.

library/compiler-builtins/README.md

@@ -240,7 +240,7 @@ of being added to Rust.
 - [ ] floatunsitf.c
 - [ ] floatuntitf.c
 - [x] multf3.c
-- [ ] powitf2.c
+- [x] powitf2.c
 - [x] subtf3.c
 - [x] truncdfhf2.c
 - [x] truncsfhf2.c

library/compiler-builtins/build.rs

@@ -526,7 +526,6 @@ mod c {
                 ("__floatsitf", "floatsitf.c"),
                 ("__floatunditf", "floatunditf.c"),
                 ("__floatunsitf", "floatunsitf.c"),
-                ("__powitf2", "powitf2.c"),
                 ("__fe_getround", "fp_mode.c"),
                 ("__fe_raise_inexact", "fp_mode.c"),
             ]);

library/compiler-builtins/src/float/pow.rs

@@ -35,4 +35,13 @@ intrinsics! {
     pub extern "C" fn __powidf2(a: f64, b: i32) -> f64 {
         pow(a, b)
     }
+
+    #[avr_skip]
+    #[ppc_alias = __powikf2]
+    #[cfg(f128_enabled)]
+    // FIXME(f16_f128): MSVC cannot build these until `__divtf3` is available in nightly.
+    #[cfg(not(target_env = "msvc"))]
+    pub extern "C" fn __powitf2(a: f128, b: i32) -> f128 {
+        pow(a, b)
+    }
 }

library/compiler-builtins/testcrate/tests/float_pow.rs

@@ -1,4 +1,5 @@
 #![allow(unused_macros)]
+#![cfg_attr(f128_enabled, feature(f128))]
 #![cfg(not(all(target_arch = "x86", not(target_feature = "sse"))))]
 
 use testcrate::*;
@@ -7,9 +8,12 @@ use testcrate::*;
 // https://github.com/rust-lang/rust/issues/73920.
 // TODO how do we resolve this indeterminacy?
 macro_rules! pow {
-    ($($f:ty, $tolerance:expr, $fn:ident);*;) => {
+    ($($f:ty, $tolerance:expr, $fn:ident, $sys_available:meta);*;) => {
         $(
             #[test]
+            // FIXME(apfloat): We skip tests if system symbols aren't available rather
+            // than providing a fallback, since `rustc_apfloat` does not provide `pow`.
+            #[cfg($sys_available)]
             fn $fn() {
                 use compiler_builtins::float::pow::$fn;
                 use compiler_builtins::float::Float;
@@ -49,6 +53,20 @@ macro_rules! pow {
 }
 
 pow! {
-    f32, 1e-4, __powisf2;
-    f64, 1e-12, __powidf2;
+    f32, 1e-4, __powisf2, all();
+    f64, 1e-12, __powidf2, all();
+}
+
+#[cfg(f128_enabled)]
+// FIXME(f16_f128): MSVC cannot build these until `__divtf3` is available in nightly.
+#[cfg(not(target_env = "msvc"))]
+#[cfg(not(any(target_arch = "powerpc", target_arch = "powerpc64")))]
+pow! {
+    f128, 1e-36, __powitf2, not(feature = "no-sys-f128");
+}
+
+#[cfg(f128_enabled)]
+#[cfg(any(target_arch = "powerpc", target_arch = "powerpc64"))]
+pow! {
+    f128, 1e-36, __powikf2, not(feature = "no-sys-f128");
 }