AK: Add a cpp-y, more fine grained version of fenv.h: FPControl.h

This allows direct inlining and hides away some assembly and
bit-fiddling when manipulating the floating point environment.

This only implements the x87/SSE versions, as of now.

1 files changed, 124 insertions, 0 deletions

diff --git a/AK/FPControl.h b/AK/FPControl.h
new file mode 100644
index 0000000000..6639338aba
--- /dev/null
+++ b/AK/FPControl.h
@@ -0,0 +1,124 @@
+/*
+ * Copyright (c) 2022, Leon Albrecht <leon.a@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+
+// FIXME: Add equivalent datastructures for aarch64
+VALIDATE_IS_X86();
+
+namespace AK {
+
+enum class RoundingMode : u8 {
+    NEAREST = 0b00,
+    DOWN = 0b01,
+    UP = 0b10,
+    TRUNC = 0b11
+};
+
+union X87ControlWord {
+    u16 cw;
+    struct {
+        u16 mask_invalid : 1;              // IM
+        u16 mask_denorm : 1;               // DM
+        u16 mask_zero_div : 1;             // ZM
+        u16 mask_overflow : 1;             // OM
+        u16 mask_underflow : 1;            // UM
+        u16 mask_precision : 1;            // PM
+        u16 : 2;                           // unused
+        u16 precision : 2;                 // PC
+        RoundingMode rounding_control : 2; // RC
+        u16 infinity_control : 1;          // X
+        u16 : 3;                           // unused
+    };
+};
+static_assert(sizeof(X87ControlWord) == sizeof(u16));
+
+union MXCSR {
+    u32 mxcsr;
+    struct {
+        u32 invalid_operation_flag : 1;    // IE
+        u32 denormal_operation_flag : 1;   // DE
+        u32 divide_by_zero_flag : 1;       // ZE
+        u32 overflow_flag : 1;             // OE
+        u32 underflow_flag : 1;            // UE
+        u32 precision_flag : 1;            // PE
+        u32 denormals_are_zero : 1;        // DAZ
+        u32 invalid_operation_mask : 1;    // IM
+        u32 denormal_operation_mask : 1;   // DM
+        u32 divide_by_zero_mask : 1;       // ZM
+        u32 overflow_mask : 1;             // OM
+        u32 underflow_mask : 1;            // UM
+        u32 precision_mask : 1;            // PM
+        RoundingMode rounding_control : 2; // RC
+        u32 flush_to_zero : 1;             // FTZ
+        u32 __reserved : 16;
+    };
+};
+static_assert(sizeof(MXCSR) == sizeof(u32));
+
+ALWAYS_INLINE X87ControlWord get_cw_x87()
+{
+    X87ControlWord control_word;
+    asm("fnstcw %0"
+        : "=m"(control_word));
+    return control_word;
+}
+ALWAYS_INLINE void set_cw_x87(X87ControlWord control_word)
+{
+    asm("fldcw %0" ::"m"(control_word));
+}
+
+ALWAYS_INLINE MXCSR get_mxcsr()
+{
+    MXCSR mxcsr;
+    asm("stmxcsr %0"
+        : "=m"(mxcsr));
+    return mxcsr;
+}
+ALWAYS_INLINE void set_mxcsr(MXCSR mxcsr)
+{
+    asm("ldmxcsr %0" ::"m"(mxcsr));
+}
+
+class X87RoundingModeScope {
+public:
+    X87RoundingModeScope(RoundingMode rounding_mode)
+    {
+        m_cw = get_cw_x87();
+        auto cw = m_cw;
+        cw.rounding_control = rounding_mode;
+        set_cw_x87(cw);
+    }
+    ~X87RoundingModeScope()
+    {
+        set_cw_x87(m_cw);
+    }
+
+private:
+    X87ControlWord m_cw;
+};
+
+class SSERoundingModeScope {
+public:
+    SSERoundingModeScope(RoundingMode rounding_mode)
+    {
+        m_mxcsr = get_mxcsr();
+        auto mxcsr = m_mxcsr;
+        mxcsr.rounding_control = rounding_mode;
+        set_mxcsr(mxcsr);
+    }
+    ~SSERoundingModeScope()
+    {
+        set_mxcsr(m_mxcsr);
+    }
+
+private:
+    MXCSR m_mxcsr;
+};
+
+}