diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 701bacb9..76e03e2f 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -252,12 +252,11 @@ jobs:
           RUSTFLAGS: ${{ env.RUSTFLAGS }} -C target-feature=-sse2
           RUSTDOCFLAGS: ${{ env.RUSTDOCFLAGS }} -C target-feature=-sse2
         if: startsWith(matrix.target, 'i686')
-      # x86 x87
+      # x86 -x87
       - run: tools/test.sh -vv --tests $TARGET $BUILD_STD $RELEASE
         env:
-          # i586 is -C target-feature=+x87 by default, but cfg(target_feature = "x87") doesn't work.
-          RUSTFLAGS: ${{ env.RUSTFLAGS }} --cfg target_feature="x87"
-          RUSTDOCFLAGS: ${{ env.RUSTDOCFLAGS }} --cfg target_feature="x87"
+          RUSTFLAGS: ${{ env.RUSTFLAGS }} -C target-feature=+soft-float,-x87,-mmx
+          RUSTDOCFLAGS: ${{ env.RUSTDOCFLAGS }} -C target-feature=+soft-float,-x87,-mmx
         if: startsWith(matrix.target, 'i586')
       # aarch64 +lse
       - run: tools/test.sh -vv --tests $TARGET $BUILD_STD $RELEASE
diff --git a/build.rs b/build.rs
index 11d7bee2..e25b171a 100644
--- a/build.rs
+++ b/build.rs
@@ -41,7 +41,7 @@ fn main() {
         // TODO: handle multi-line target_feature_fallback
         // grep -E 'target_feature_fallback\("' build.rs | sed -E 's/^.*target_feature_fallback\(//; s/",.*$/"/' | LC_ALL=C sort -u | tr '\n' ','
         println!(
-            r#"cargo:rustc-check-cfg=cfg(atomic_maybe_uninit_target_feature,values("a","cmpxchg16b","fast-serialization","lse","lse128","lse2","mclass","partword-atomics","quadword-atomics","rcpc","rcpc3","v5te","v6","v7","v8","v8m"))"#
+            r#"cargo:rustc-check-cfg=cfg(atomic_maybe_uninit_target_feature,values("a","cmpxchg16b","fast-serialization","lse","lse128","lse2","mclass","partword-atomics","quadword-atomics","rcpc","rcpc3","v5te","v6","v7","v8","v8m","x87"))"#
         );
     }
 
@@ -108,6 +108,15 @@ fn main() {
         || target_os == "watchos"
         || target_os == "visionos";
     match target_arch {
+        "x86" => {
+            // i586 is -C target-feature=+x87 by default, but cfg(target_feature = "x87") doesn't work.
+            // And core assumes x87 is always available.
+            // https://github.com/rust-lang/rust/blob/1.80.0/library/core/src/num/dec2flt/fpu.rs#L6
+            // https://github.com/rust-lang/rust/blob/1.80.0/compiler/rustc_target/src/spec/targets/i686_unknown_uefi.rs#L24
+            // However, custom bare metal targets tend to disable x87 and do not use floats.
+            let has_x87 = target_os != "none";
+            target_feature_fallback("x87", has_x87);
+        }
         "x86_64" => {
             // cmpxchg16b_target_feature stabilized in Rust 1.69.
             if needs_target_feature_fallback(&version, Some(69)) {
diff --git a/src/arch/x86.rs b/src/arch/x86.rs
index f1d187b7..8b52df86 100644
--- a/src/arch/x86.rs
+++ b/src/arch/x86.rs
@@ -248,9 +248,7 @@ macro_rules! atomic64 {
                     core::mem::transmute::<_, [MaybeUninit<Self>; 2]>(out)[0]
                 }
                 #[cfg(not(target_feature = "sse"))]
-                // It seems core assumes x87 target_feature is always available?
-                // https://github.com/rust-lang/rust/blob/1.80.0/library/core/src/num/dec2flt/fpu.rs#L6
-                #[cfg(target_feature = "x87")]
+                #[cfg(any(target_feature = "x87", atomic_maybe_uninit_target_feature = "x87"))]
                 // SAFETY: the caller must uphold the safety contract.
                 //
                 // Refs:
@@ -279,7 +277,7 @@ macro_rules! atomic64 {
                     out
                 }
                 #[cfg(not(target_feature = "sse"))]
-                #[cfg(not(target_feature = "x87"))]
+                #[cfg(not(any(target_feature = "x87", atomic_maybe_uninit_target_feature = "x87")))]
                 // SAFETY: the caller must uphold the safety contract.
                 //
                 // Refs: https://www.felixcloutier.com/x86/cmpxchg8b:cmpxchg16b
@@ -348,9 +346,7 @@ macro_rules! atomic64 {
                     }
                 }
                 #[cfg(not(target_feature = "sse"))]
-                // It seems core assumes x87 target_feature is always available?
-                // https://github.com/rust-lang/rust/blob/1.80.0/library/core/src/num/dec2flt/fpu.rs#L6
-                #[cfg(target_feature = "x87")]
+                #[cfg(any(target_feature = "x87", atomic_maybe_uninit_target_feature = "x87"))]
                 // SAFETY: the caller must uphold the safety contract.
                 //
                 // Refs:
@@ -402,7 +398,7 @@ macro_rules! atomic64 {
                     }
                 }
                 #[cfg(not(target_feature = "sse"))]
-                #[cfg(not(target_feature = "x87"))]
+                #[cfg(not(any(target_feature = "x87", atomic_maybe_uninit_target_feature = "x87")))]
                 // SAFETY: the caller must uphold the safety contract.
                 //
                 // Refs: https://www.felixcloutier.com/x86/cmpxchg8b:cmpxchg16b
diff --git a/tools/build.sh b/tools/build.sh
index c110a83b..65b2c363 100755
--- a/tools/build.sh
+++ b/tools/build.sh
@@ -240,9 +240,8 @@ build() {
                 x_cargo "${args[@]}" "$@"
             ;;
         i586*)
-            # i586 is -C target-feature=+x87 by default, but cfg(target_feature = "x87") doesn't work.
-            CARGO_TARGET_DIR="${target_dir}/x87" \
-                RUSTFLAGS="${target_rustflags} --cfg target_feature=\"x87\"" \
+            CARGO_TARGET_DIR="${target_dir}/no-x87" \
+                RUSTFLAGS="${target_rustflags} -C target-feature=-x87" \
                 x_cargo "${args[@]}" "$@"
             ;;
         aarch64* | arm64*)