ran-j · drakolordx7 · Sep 30, 2026
diff --git a/ps2xRecomp/src/lib/fpu_translator.cpp b/ps2xRecomp/src/lib/fpu_translator.cpp
@@ -53,10 +53,10 @@ namespace ps2recomp
             case COP1_S_MUL:
                 return fmt::format("ctx->f[{}] = FPU_MUL_S(ctx->f[{}], ctx->f[{}]);", fd, fs, ft);
             case COP1_S_DIV:
-                return fmt::format("if (ctx->f[{}] == 0.0f) {{ ctx->fcr31 |= 0x100000; /* DZ flag */ "
-                                   "ctx->f[{}] = copysignf(INFINITY, ctx->f[{}] * 0.0f); }} "
-                                   "else ctx->f[{}] = ctx->f[{}] / ctx->f[{}];",
-                                   ft, fd, fs, fd, fs, ft);
+                // The EE has no Inf/NaN: x/0 gives +-max (sign of fs xor ft), see ps2_fdiv.
+                return fmt::format("{{ if (ctx->f[{}] == 0.0f) ctx->fcr31 |= 0x100000; /* DZ flag */ "
+                                   "ctx->f[{}] = FPU_DIV_S(ctx->f[{}], ctx->f[{}]); }}",
+                                   ft, fd, fs, ft);
             case COP1_S_SQRT:
                 return fmt::format("ctx->f[{}] = FPU_SQRT_S(ctx->f[{}]);", fd, fs);
             case COP1_S_ABS:
@@ -76,7 +76,8 @@ namespace ps2recomp
             case COP1_S_CVT_W:
                 return fmt::format("{{ int32_t tmp = FPU_CVT_W_S(ctx->f[{}]); std::memcpy(&ctx->f[{}], &tmp, sizeof(tmp)); }}", fs, fd);
             case COP1_S_RSQRT:
-                return fmt::format("ctx->f[{}] = 1.0f / sqrtf(ctx->f[{}]);", fd, fs);
+                // RSQRT.S fd, fs, ft computes fs / sqrt(ft), not 1 / sqrt(fs).
+                return fmt::format("ctx->f[{}] = FPU_RSQRT_S(ctx->f[{}], ctx->f[{}]);", fd, fs, ft);
             case COP1_S_ADDA:
                 return fmt::format("FPU_SET_ACC(ctx, FPU_ADD_S(ctx->f[{}], ctx->f[{}]));", fs, ft);
             case COP1_S_SUBA:

diff --git a/ps2xRuntime/include/ps2_runtime_macros.h b/ps2xRuntime/include/ps2_runtime_macros.h
@@ -140,10 +140,60 @@ static inline uint32_t ps2_plzcw32(uint32_t x)
 #define PS2_PXOR(a, b) _mm_xor_si128((__m128i)(a), (__m128i)(b))
 #define PS2_PNOR(a, b) _mm_xor_si128(_mm_or_si128((__m128i)(a), (__m128i)(b)), _mm_set1_epi32(0xFFFFFFFF))
 
+// PS2 float semantics (EE FPU and VU). The PS2 has no Inf/NaN: a value with exponent 255 behaves like a huge
+// normal number, an overflowing result saturates to +-max (0x7F7FFFFF) and x/0 gives +-max with the sign of
+// (numerator xor denominator). Hosts that follow IEEE produce Inf/NaN here, which then poisons later maths
+// (e.g. a normalisation of a zero-length vector).
+inline float ps2_fclamp(float x)
+{
+    uint32_t b;
+    std::memcpy(&b, &x, sizeof(b));
+    if ((b & 0x7F800000u) == 0x7F800000u)
+        b = (b & 0x80000000u) | 0x7F7FFFFFu;
+    std::memcpy(&x, &b, sizeof(b));
+    return x;
+}
+
+// Sign is sign(a) xor sign(b); used for a division by zero.
+inline float ps2_fmax_signed(float a, float b)
+{
+    uint32_t ua, ub;
+    std::memcpy(&ua, &a, sizeof(ua));
+    std::memcpy(&ub, &b, sizeof(ub));
+    const uint32_t r = ((ua ^ ub) & 0x80000000u) | 0x7F7FFFFFu;
+    float f;
+    std::memcpy(&f, &r, sizeof(f));
+    return f;
+}
+
+inline float ps2_fdiv(float a, float b)
+{
+    a = ps2_fclamp(a);
+    b = ps2_fclamp(b);
+    return (b == 0.0f) ? ps2_fmax_signed(a, b) : ps2_fclamp(a / b);
+}
+
+// fs / sqrt(|ft|); the radicand's sign is ignored.
+inline float ps2_frsqrt(float a, float b)
+{
+    a = ps2_fclamp(a);
+    b = ps2_fclamp(b);
+    return (b == 0.0f) ? ps2_fmax_signed(a, b) : ps2_fclamp(a / std::sqrt(std::fabs(b)));
+}
+
+inline __m128 ps2_vclamp(__m128 v)
+{
+    const __m128i bits = _mm_castps_si128(v);
+    const __m128i expMask = _mm_set1_epi32(0x7F800000);
+    const __m128i special = _mm_cmpeq_epi32(_mm_and_si128(bits, expMask), expMask);
+    const __m128i saturated = _mm_or_si128(_mm_and_si128(bits, _mm_set1_epi32((int)0x80000000)), _mm_set1_epi32(0x7F7FFFFF));
+    return _mm_castsi128_ps(_mm_or_si128(_mm_and_si128(special, saturated), _mm_andnot_si128(special, bits)));
+}
+
 // PS2 VU (Vector Unit) operations
-#define PS2_VADD(a, b) _mm_add_ps((__m128)(a), (__m128)(b))
-#define PS2_VSUB(a, b) _mm_sub_ps((__m128)(a), (__m128)(b))
-#define PS2_VMUL(a, b) _mm_mul_ps((__m128)(a), (__m128)(b))
+#define PS2_VADD(a, b) ps2_vclamp(_mm_add_ps(ps2_vclamp((__m128)(a)), ps2_vclamp((__m128)(b))))
+#define PS2_VSUB(a, b) ps2_vclamp(_mm_sub_ps(ps2_vclamp((__m128)(a)), ps2_vclamp((__m128)(b))))
+#define PS2_VMUL(a, b) ps2_vclamp(_mm_mul_ps(ps2_vclamp((__m128)(a)), ps2_vclamp((__m128)(b))))
 #define PS2_VDIV(a, b) _mm_div_ps((__m128)(a), (__m128)(b))
 #define PS2_VMULQ(a, q) _mm_mul_ps((__m128)(a), _mm_set1_ps(q))
 #define PS2_VBLEND(a, b, mask) PS2_BLENDV_PS((__m128)(a), (__m128)(b), (__m128)(mask))
@@ -605,11 +655,12 @@ inline __m128i ps2_u64_to_epi64_pair(uint64_t value)
 
 // FPU (COP1) operations
 #define FPU_SET_ACC(ctx, res) (ctx->f_acc = res)
-#define FPU_ADD_S(a, b) ((float)(a) + (float)(b))
-#define FPU_SUB_S(a, b) ((float)(a) - (float)(b))
-#define FPU_MUL_S(a, b) ((float)(a) * (float)(b))
-#define FPU_DIV_S(a, b) ((float)(a) / (float)(b))
-#define FPU_SQRT_S(a) sqrtf((float)(a))
+#define FPU_ADD_S(a, b) ps2_fclamp(ps2_fclamp((float)(a)) + ps2_fclamp((float)(b)))
+#define FPU_SUB_S(a, b) ps2_fclamp(ps2_fclamp((float)(a)) - ps2_fclamp((float)(b)))
+#define FPU_MUL_S(a, b) ps2_fclamp(ps2_fclamp((float)(a)) * ps2_fclamp((float)(b)))
+#define FPU_DIV_S(a, b) ps2_fdiv((float)(a), (float)(b))
+#define FPU_RSQRT_S(a, b) ps2_frsqrt((float)(a), (float)(b))
+#define FPU_SQRT_S(a) std::sqrt(std::fabs(ps2_fclamp((float)(a))))
 #define FPU_ABS_S(a) fabsf((float)(a))
 #define FPU_MOV_S(a) ((float)(a))
 #define FPU_NEG_S(a) (-(float)(a))

diff --git a/ps2xTest/CMakeLists.txt b/ps2xTest/CMakeLists.txt
@@ -37,6 +37,7 @@ add_library(ps2_test_lib STATIC
     src/ps2_sif_dma_tests.cpp
     src/ps2_recompiler_tests.cpp
     src/ps2_runtime_expansion_tests.cpp
+    src/ps2_float_semantics_tests.cpp
 )
 
 target_include_directories(ps2_test_lib PRIVATE

diff --git a/ps2xTest/src/main.cpp b/ps2xTest/src/main.cpp
@@ -18,6 +18,7 @@ void register_ps2_sif_rpc_tests();
 void register_ps2_sif_dma_tests();
 void register_ps2_recompiler_tests();
 void register_ps2_runtime_expansion_tests();
+void register_ps2_float_semantics_tests();
 void reset_ps2_test_function_table();
 
 int main()
@@ -40,6 +41,7 @@ int main()
     register_ps2_sif_dma_tests();
     register_ps2_recompiler_tests();
     register_ps2_runtime_expansion_tests();
+    register_ps2_float_semantics_tests();
     int res = MiniTest::Run();
     std::cout.flush();
     std::cerr.flush();

diff --git a/ps2xTest/src/ps2_float_semantics_tests.cpp b/ps2xTest/src/ps2_float_semantics_tests.cpp
@@ -0,0 +1,157 @@
+#include "MiniTest.h"
+#include "ps2recomp/code_generator.h"
+#include "ps2recomp/instructions.h"
+#include "ps2recomp/types.h"
+#include "ps2_runtime_macros.h"
+
+#include <cmath>
+#include <cstdint>
+#include <cstring>
+#include <limits>
+#include <string>
+
+using namespace ps2recomp;
+
+namespace
+{
+    constexpr uint32_t kPlusMax = 0x7F7FFFFFu;
+    constexpr uint32_t kMinusMax = 0xFF7FFFFFu;
+
+    uint32_t bitsOf(float f)
+    {
+        uint32_t b;
+        std::memcpy(&b, &f, sizeof(b));
+        return b;
+    }
+
+    float floatOf(uint32_t b)
+    {
+        float f;
+        std::memcpy(&f, &b, sizeof(f));
+        return f;
+    }
+
+    Instruction makeCop1S(uint32_t function, uint8_t fd, uint8_t fs, uint8_t ft)
+    {
+        Instruction inst{};
+        inst.opcode = OPCODE_COP1;
+        inst.rs = COP1_S;
+        inst.rt = ft;
+        inst.rd = fs;
+        inst.sa = fd;
+        inst.function = function;
+        return inst;
+    }
+
+    uint32_t laneBits(__m128 v, int lane)
+    {
+        float lanes[4];
+        _mm_storeu_ps(lanes, v);
+        return bitsOf(lanes[lane]);
+    }
+}
+
+void register_ps2_float_semantics_tests()
+{
+    MiniTest::Case("PS2FloatSemantics", [](TestCase &tc)
+    {
+        tc.Run("ps2_fclamp turns Inf and NaN into +-max and leaves finite values alone", [](TestCase &t)
+        {
+            t.Equals(bitsOf(ps2_fclamp(std::numeric_limits<float>::infinity())), kPlusMax, "+Inf should become +max");
+            t.Equals(bitsOf(ps2_fclamp(-std::numeric_limits<float>::infinity())), kMinusMax, "-Inf should become -max");
+            t.Equals(bitsOf(ps2_fclamp(floatOf(0x7FC00000u))), kPlusMax, "positive NaN should become +max");
+            t.Equals(bitsOf(ps2_fclamp(floatOf(0xFFC00000u))), kMinusMax, "negative NaN should become -max");
+            t.Equals(bitsOf(ps2_fclamp(1.5f)), bitsOf(1.5f), "finite values should be unchanged");
+            t.Equals(bitsOf(ps2_fclamp(floatOf(kPlusMax))), kPlusMax, "max should be unchanged");
+            t.Equals(bitsOf(ps2_fclamp(-0.0f)), bitsOf(-0.0f), "negative zero should be unchanged");
+        });
+
+        tc.Run("FPU ADD/SUB/MUL saturate instead of overflowing to Inf", [](TestCase &t)
+        {
+            const float maxf = floatOf(kPlusMax);
+            const float minf = floatOf(kMinusMax);
+            t.Equals(bitsOf(FPU_ADD_S(maxf, maxf)), kPlusMax, "max + max should saturate to +max");
+            t.Equals(bitsOf(FPU_ADD_S(minf, minf)), kMinusMax, "-max + -max should saturate to -max");
+            t.Equals(bitsOf(FPU_SUB_S(maxf, minf)), kPlusMax, "max - (-max) should saturate to +max");
+            t.Equals(bitsOf(FPU_MUL_S(maxf, 2.0f)), kPlusMax, "max * 2 should saturate to +max");
+            t.Equals(bitsOf(FPU_MUL_S(maxf, -2.0f)), kMinusMax, "max * -2 should saturate to -max");
+            t.Equals(bitsOf(FPU_ADD_S(1.0f, 2.0f)), bitsOf(3.0f), "ordinary addition should be unchanged");
+            t.Equals(bitsOf(FPU_MUL_S(1.5f, 4.0f)), bitsOf(6.0f), "ordinary multiplication should be unchanged");
+        });
+
+        tc.Run("FPU ADD/SUB/MUL never produce NaN from Inf or NaN operands", [](TestCase &t)
+        {
+            const float inf = std::numeric_limits<float>::infinity();
+            const float nanf = floatOf(0x7FC00000u);
+            t.IsFalse(std::isnan(FPU_SUB_S(inf, inf)), "Inf - Inf should not be NaN");
+            t.IsFalse(std::isnan(FPU_MUL_S(inf, 0.0f)), "Inf * 0 should not be NaN");
+            t.Equals(bitsOf(FPU_MUL_S(inf, 0.0f)), bitsOf(0.0f), "Inf * 0 should be 0");
+            t.IsFalse(std::isnan(FPU_ADD_S(nanf, 1.0f)), "NaN input should not produce NaN");
+            t.IsFalse(std::isinf(FPU_ADD_S(nanf, 1.0f)), "NaN input should not leave an Inf");
+        });
+
+        tc.Run("FPU DIV.S by zero gives +-max with the sign of fs xor ft", [](TestCase &t)
+        {
+            t.Equals(bitsOf(FPU_DIV_S(1.0f, 0.0f)), kPlusMax, "1/+0 should be +max");
+            t.Equals(bitsOf(FPU_DIV_S(-1.0f, 0.0f)), kMinusMax, "-1/+0 should be -max");
+            t.Equals(bitsOf(FPU_DIV_S(1.0f, -0.0f)), kMinusMax, "1/-0 should be -max");
+            t.Equals(bitsOf(FPU_DIV_S(-1.0f, -0.0f)), kPlusMax, "-1/-0 should be +max");
+            t.Equals(bitsOf(FPU_DIV_S(0.0f, 0.0f)), kPlusMax, "0/0 should be +max, not NaN");
+            t.Equals(bitsOf(FPU_DIV_S(6.0f, 3.0f)), bitsOf(2.0f), "ordinary division should be unchanged");
+            t.Equals(bitsOf(FPU_DIV_S(floatOf(kPlusMax), 0.5f)), kPlusMax, "overflowing quotient should saturate");
+        });
+
+        tc.Run("FPU RSQRT.S computes fs / sqrt(ft)", [](TestCase &t)
+        {
+            t.Equals(bitsOf(FPU_RSQRT_S(4.0f, 16.0f)), bitsOf(1.0f), "4 / sqrt(16) should be 1");
+            t.Equals(bitsOf(FPU_RSQRT_S(1.0f, 4.0f)), bitsOf(0.5f), "1 / sqrt(4) should be 0.5");
+            t.Equals(bitsOf(FPU_RSQRT_S(1.0f, -4.0f)), bitsOf(0.5f), "the sign of the radicand should be ignored");
+            t.Equals(bitsOf(FPU_RSQRT_S(2.0f, 0.0f)), kPlusMax, "x / sqrt(0) should be +max");
+            t.Equals(bitsOf(FPU_RSQRT_S(-2.0f, 0.0f)), kMinusMax, "-x / sqrt(0) should be -max");
+        });
+
+        tc.Run("FPU SQRT.S takes the square root of the magnitude", [](TestCase &t)
+        {
+            t.Equals(bitsOf(FPU_SQRT_S(16.0f)), bitsOf(4.0f), "sqrt(16) should be 4");
+            t.Equals(bitsOf(FPU_SQRT_S(-16.0f)), bitsOf(4.0f), "sqrt(-16) should be 4, not NaN");
+        });
+
+        tc.Run("VU vector ADD/SUB/MUL clamp every lane", [](TestCase &t)
+        {
+            // _mm_set_ps lists lanes 3..0
+            const __m128 a = _mm_set_ps(1.0f, std::numeric_limits<float>::infinity(), floatOf(kMinusMax), floatOf(kPlusMax));
+            const __m128 b = _mm_set_ps(2.0f, 1.0f, floatOf(kMinusMax), floatOf(kPlusMax));
+
+            const __m128 sum = PS2_VADD(a, b);
+            t.Equals(laneBits(sum, 0), kPlusMax, "lane 0: max + max should saturate to +max");
+            t.Equals(laneBits(sum, 1), kMinusMax, "lane 1: -max + -max should saturate to -max");
+            t.Equals(laneBits(sum, 2), kPlusMax, "lane 2: +Inf + 1 should be +max");
+            t.Equals(laneBits(sum, 3), bitsOf(3.0f), "lane 3: 1 + 2 should be 3");
+
+            const __m128 diff = PS2_VSUB(a, b);
+            t.Equals(laneBits(diff, 0), bitsOf(0.0f), "lane 0: max - max should be 0");
+            t.Equals(laneBits(diff, 3), bitsOf(-1.0f), "lane 3: 1 - 2 should be -1");
+
+            const __m128 prod = PS2_VMUL(a, b);
+            t.Equals(laneBits(prod, 0), kPlusMax, "lane 0: max * max should saturate to +max");
+            t.Equals(laneBits(prod, 1), kPlusMax, "lane 1: -max * -max should saturate to +max");
+            t.Equals(laneBits(prod, 2), kPlusMax, "lane 2: +Inf * 1 should be +max");
+            t.Equals(laneBits(prod, 3), bitsOf(2.0f), "lane 3: 1 * 2 should be 2");
+        });
+
+        tc.Run("DIV.S and RSQRT.S translate to the PS2 semantics helpers", [](TestCase &t)
+        {
+            CodeGenerator gen({}, {});
+
+            // fd = f2, fs = f3, ft = f4 (all distinct so a wrong operand shows up)
+            const std::string div = gen.translateInstruction(makeCop1S(COP1_S_DIV, 2, 3, 4));
+            t.IsTrue(div.find("ctx->f[2] = FPU_DIV_S(ctx->f[3], ctx->f[4])") != std::string::npos,
+                     "DIV.S should write fs / ft to fd but got: " + div);
+            t.IsTrue(div.find("INFINITY") == std::string::npos, "DIV.S must not produce IEEE infinity but got: " + div);
+
+            const std::string rsqrt = gen.translateInstruction(makeCop1S(COP1_S_RSQRT, 2, 3, 4));
+            t.IsTrue(rsqrt.find("ctx->f[2] = FPU_RSQRT_S(ctx->f[3], ctx->f[4])") != std::string::npos,
+                     "RSQRT.S should compute fs / sqrt(ft) but got: " + rsqrt);
+        });
+    });
+}