Enable MPFR tests on i586

ci/docker/i586-unknown-linux-gnu/Dockerfile

@@ -2,4 +2,4 @@ FROM ubuntu:24.04
 
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
-    gcc-multilib libc6-dev ca-certificates
+    gcc-multilib m4 make libc6-dev ca-certificates

ci/run.sh

@@ -45,8 +45,8 @@ case "$target" in
     # FIXME(ci): we should be able to enable aarch64 Linux here once GHA
     # support rolls out.
     x86_64*) extra_flags="$extra_flags --features libm-test/test-multiprecision" ;;
-    # i686 works fine, i586 does not
     i686*) extra_flags="$extra_flags --features libm-test/test-multiprecision" ;;
+    i586*) extra_flags="$extra_flags --features libm-test/test-multiprecision --features gmp-mpfr-sys/force-cross" ;;
     # Apple aarch64 is native
     aarch64*apple*) extra_flags="$extra_flags --features libm-test/test-multiprecision" ;;
 esac

crates/libm-test/Cargo.toml

@@ -12,7 +12,7 @@ unstable-float = ["libm/unstable-float", "rug?/nightly-float"]
 
 # Generate tests which are random inputs and the outputs are calculated with
 # musl libc.
-test-multiprecision = ["dep:az", "dep:rug"]
+test-multiprecision = ["dep:az", "dep:rug", "dep:gmp-mpfr-sys"]
 
 # Build our own musl for testing and benchmarks
 build-musl = ["dep:musl-math-sys"]
@@ -26,6 +26,7 @@ short-benchmarks = []
 [dependencies]
 anyhow = "1.0.90"
 az = { version = "1.2.1", optional = true }
+gmp-mpfr-sys = { version = "1.6.4", optional = true, default-features = false, features = ["mpfr"] }
 indicatif = { version = "0.17.9", default-features = false }
 libm = { path = "../..", features = ["unstable-public-internals"] }
 libm-macros = { path = "../libm-macros" }

crates/libm-test/src/precision.rs

@@ -104,11 +104,14 @@ pub fn default_ulp(ctx: &CheckCtx) -> u32 {
     // In some cases, our implementation is less accurate than musl on i586.
     if cfg!(x86_no_sse) {
         match ctx.fn_ident {
+            // FIXME(#401): these need to be correctly rounded but are not.
+            Id::Fmaf => ulp = 1,
+            Id::Fdim => ulp = 1,
+
             Id::Asinh => ulp = 3,
             Id::Asinhf => ulp = 3,
             Id::Exp10 | Id::Exp10f => ulp = 1_000_000,
             Id::Exp2 | Id::Exp2f => ulp = 10_000_000,
-            Id::Fmaf => ulp = 1,
             Id::Log1p | Id::Log1pf => ulp = 2,
             Id::Rint => ulp = 100_000,
             Id::Round => ulp = 1,