Fix FScale'ing zero that should return zero

Added tests will fail without the current patch.
This will properly check if we return the correct sign of zero.

FEXCore/Source/Common/SoftFloat.h

@@ -262,6 +262,10 @@ struct FEX_PACKED X80SoftFloat {
 
     return Result;
 #else
+    extFloat80_t Zero {0, 0};
+    if (extF80_eq(state, lhs, Zero)) {
+      return lhs;
+    }
     X80SoftFloat Int = FRNDINT(state, rhs, softfloat_round_minMag);
     LIBRARY_PRECISION Src2_d = Int.ToFMax(state);
     Src2_d = exp2l(Src2_d);

FEXCore/Source/Interface/Core/Interpreter/Fallbacks/F80Fallbacks.h

@@ -322,8 +322,11 @@ struct OpHandlers<IR::OP_F64FYL2X> {
 template<>
 struct OpHandlers<IR::OP_F64SCALE> {
   static double handle(uint16_t FCW, double src1, double src2) {
-    double trunc = (double)(int64_t)(src2); // truncate
-    return src1 * exp2(trunc);
+    if (src1 == 0.0) {
+      return std::signbit(src1) ? -0.0 : 0.0;
+    }
+    double trun = trunc(src2);
+    return src1 * exp2(trun);
   }
 };
 

unittests/ASM/X87/FScale-Zero.asm

@@ -0,0 +1,85 @@
+%ifdef CONFIG
+{
+  "RegData": {
+    "R8": "0",
+    "R9": "0",
+    "R10": "0",
+    "R11": "0",
+    "R12": "0",
+    "R13": "0x8000000000000000"
+  }
+}
+%endif
+
+section .data
+    neg_zero dq 0x8000000000000000   ; -0.0
+
+section .bss
+  align 8
+  intstor resq 1
+
+section .text
+global _start
+
+_start:
+; scale by zero (st1 == 0)
+mov rax, 0
+mov qword [rel intstor], rax
+finit
+fild qword [rel intstor]
+fldz
+fscale
+fst qword [rel intstor]
+mov r8, [rel intstor]
+
+; scale by zero (st1 == 1)
+mov rax, 1
+mov qword [rel intstor], rax
+finit
+fild qword [rel intstor]
+fldz
+fscale
+fst qword [rel intstor]
+mov r9, [rel intstor]
+
+; scale by zero (st1 == 100)
+mov rax, 100
+mov qword [rel intstor], rax
+finit
+fild qword [rel intstor]
+fldz
+fscale
+fst qword [rel intstor]
+mov r10, [rel intstor]
+
+; scale by zero (st1 == 1024)
+mov rax, 1024
+mov qword [rel intstor], rax
+finit
+fild qword [rel intstor]
+fldz
+fscale
+fst qword [rel intstor]
+mov r11, [rel intstor]
+
+; scale by zero (st1 == 1048576)
+mov rax, 1048576
+mov qword [rel intstor], rax
+finit
+fild qword [rel intstor]
+fldz
+fscale
+fst qword [rel intstor]
+mov r12, [rel intstor]
+
+; tests scaling negative zero
+mov rax, 1048576
+mov qword [rel intstor], rax
+finit
+fild qword [rel intstor]
+fld qword [rel neg_zero]
+fscale
+fst qword [rel intstor]
+mov r13, [rel intstor]
+
+hlt

unittests/ASM/X87_F64/FScale-Zero_F64.asm

@@ -0,0 +1,81 @@
+%ifdef CONFIG
+{
+  "RegData": {
+    "R8": "0",
+    "R9": "0",
+    "R10": "0",
+    "R11": "0",
+    "R12": "0"
+  },
+  "Env": { "FEX_X87REDUCEDPRECISION" : "1" }
+}
+%endif
+
+section .data
+    neg_zero dq 0x8000000000000000   ; -0.0
+
+section .bss
+    intstor: resq 1
+
+section .text
+; scale by zero (st1 == 0)
+mov rax, 0
+mov qword [rel intstor], rax
+finit
+fild qword [rel intstor]
+fldz
+fscale
+fst qword [rel intstor]
+mov r8, [rel intstor]
+
+; scale by zero (st1 == 1)
+mov rax, 1
+mov qword [rel intstor], rax
+finit
+fild qword [rel intstor]
+fldz
+fscale
+fst qword [rel intstor]
+mov r9, [rel intstor]
+
+; scale by zero (st1 == 100)
+mov rax, 100
+mov qword [rel intstor], rax
+finit
+fild qword [rel intstor]
+fldz
+fscale
+fst qword [rel intstor]
+mov r10, [rel intstor]
+
+; scale by zero (st1 == 1024)
+mov rax, 1024
+mov qword [rel intstor], rax
+finit
+fild qword [rel intstor]
+fldz
+fscale
+fst qword [rel intstor]
+mov r11, [rel intstor]
+
+; scale by zero (st1 == 1048576)
+mov rax, 1048576
+mov qword [rel intstor], rax
+finit
+fild qword [rel intstor]
+fldz
+fscale
+fst qword [rel intstor]
+mov r12, [rel intstor]
+
+; tests scaling negative zero
+mov rax, 1048576
+mov qword [rel intstor], rax
+finit
+fild qword [rel intstor]
+fld qword [rel neg_zero]
+fscale
+fst qword [rel intstor]
+mov r13, [rel intstor]
+
+hlt