Avoid emulating wider atomics

compiler/rustc_target/src/spec/xtensa_esp32_espidf.rs

@@ -20,12 +20,11 @@ pub fn target() -> Target {
             cpu: "esp32".into(),
             linker: Some("xtensa-esp32-elf-gcc".into()),
 
-            // The esp32 only supports native 32bit atomics. However, esp-idf will emulate 64bit atomics 
-            // so we claim a max atomic width of 64 here.
-            max_atomic_width: Some(64),
+            // esp-idf can emulate 64-bit atomics, but Rust eschews non-hardware atomics
+            max_atomic_width: Some(32),
             atomic_cas: true,
 
             ..super::xtensa_base::opts()
         },
     }
-}
+}

compiler/rustc_target/src/spec/xtensa_esp32s2_espidf.rs

@@ -22,18 +22,15 @@ pub fn target() -> Target {
 
             // See https://github.com/espressif/rust-esp32-example/issues/3#issuecomment-861054477
             //
-            // Unlike the original ESP32 chip, ESP32-S2 does not really support atomics.
-            // If the missing hardware instruction ends up being emulated in ESP-IDF, we might want to revert
-            // this change and claim that atomics are supported "in hardware" (even though they would be emulated
-            // by actually trapping the illegal instruction exception handler and calling into an ESP-IDF C emulation code).
+            // While the ESP32-S2 chip does not natively support atomics, ESP-IDF does support
+            // the __atomic* and __sync* compiler builtins. Setting `max_atomic_width` and `atomic_cas`
+            // and `atomic_cas: true` will cause the compiler to emit libcalls to these builtins.
             //
-            // However, for now we simultaneously claim "max_atomic_width: Some(64)" **and** atomic_cas: true,
-            // which should force the compiler to generate libcalls to functions that emulate atomics
-            // and which are already implemented in the ESP-IDF main branch anyway.
-            max_atomic_width: Some(64),
+            // Support for atomics is necessary for the Rust STD library, which is supported by ESP-IDF.
+            max_atomic_width: Some(32),
             atomic_cas: true,
 
             ..super::xtensa_base::opts()
         },
     }
-}
+}

compiler/rustc_target/src/spec/xtensa_esp32s3_espidf.rs

@@ -20,12 +20,11 @@ pub fn target() -> Target {
             cpu: "esp32-s3".into(),
             linker: Some("xtensa-esp32s3-elf-gcc".into()),
 
-            // The esp32s3 only supports native 32bit atomics. However, esp-idf will emulate 64bit atomics 
-            // so we claim a max atomic width of 64 here.
-            max_atomic_width: Some(64),
+            // esp-idf can emulate 64-bit atomics, but Rust eschews non-hardware atomics
+            max_atomic_width: Some(32),
             atomic_cas: true,
 
             ..super::xtensa_base::opts()
         },
     }
-}
+}