Update CmpResult to use a pointer-sized return type

As seen at [1], LLVM uses `long long` on LLP64 (to get a 64-bit integer
matching pointer size) and `long` on everything else, with exceptions
for AArch64 and AVR. Our current logic always uses an `i32`. This
happens to work because LLVM uses 32-bit instructions to check the
output on x86-64, but the GCC checks the full 64-bit register so garbage
in the upper half leads to incorrect results.

Update our return type to be `isize`, with exceptions for AArch64 and
AVR.

Fixes: rust-lang#919

[1]: https://github.com/llvm/llvm-project/blob/0cf3c437c18ed27d9663d87804a9a15ff6874af2/compiler-rt/lib/builtins/fp_compare_impl.inc#L11-L27

Author: tgross35

compiler-builtins/src/float/cmp.rs

@@ -2,14 +2,23 @@
 
 use crate::float::Float;
 use crate::int::MinInt;
-
-// https://github.com/llvm/llvm-project/blob/1e6ba3cd2fe96be00b6ed6ba28b3d9f9271d784d/compiler-rt/lib/builtins/fp_compare_impl.inc#L22
-#[cfg(target_arch = "avr")]
-pub type CmpResult = i8;
-
-// https://github.com/llvm/llvm-project/blob/1e6ba3cd2fe96be00b6ed6ba28b3d9f9271d784d/compiler-rt/lib/builtins/fp_compare_impl.inc#L25
-#[cfg(not(target_arch = "avr"))]
-pub type CmpResult = i32;
+use crate::support::cfg_if;
+
+// Taken from LLVM config:
+// https://github.com/llvm/llvm-project/blob/0cf3c437c18ed27d9663d87804a9a15ff6874af2/compiler-rt/lib/builtins/fp_compare_impl.inc#L11-L27
+cfg_if! {
+    if #[cfg(any(target_arch = "aarch64", target_arch = "arm64ec"))] {
+        // Aarch64 uses `int` rather than a pointer-sized value.
+        pub type CmpResult = i32;
+    } else if #[cfg(target_arch = "avr")] {
+        // AVR uses a single byte.
+        pub type CmpResult = i8;
+    } else {
+        // In compiler-rt, LLP64 ABIs use `long long` and everything else uses `long`. In effect,
+        // this means the return value is always pointer-sized.
+        pub type CmpResult = isize;
+    }
+}
 
 #[derive(Clone, Copy)]
 enum Result {

libm/src/math/support/mod.rs

@@ -11,6 +11,8 @@ mod int_traits;
 
 #[allow(unused_imports)]
 pub use big::{i256, u256};
+#[allow(unused_imports)]
+pub(crate) use cfg_if;
 pub use env::{FpResult, Round, Status};
 #[allow(unused_imports)]
 pub use float_traits::{DFloat, Float, HFloat, IntTy};