MDK2 fade bug

[leilei-]

Describe the bug
The scene fading function in the game cannot fade in the scene from black, causing MDK2 to not render anything. This also affects results for the benchmark funcitonality.

To Reproduce
Multiple ways:

• Start any level, it's black until you zoom-in (as Kurt) or access the main menu.
• Save/load any game.
• Start the benchmark.

Expected behavior
The scene fading in.

Screenshots

Emulator configuration

• HOT-557
• CPU: Pentium, K6/2, Winchip
• Graphics/sound cards: 3dfx Voodoo3, S3 ViRGE, Mystique, etc
• Installed OS: Windows 98SE

Host machine

• OS: Windows 7
• Emulator version: v17

Additional context
I believe it is a CPU bug, as the problem also occurs with SGI software OpenGL, Techland MSI, etc.

[sarah-walker-pcem]

Initial investigation reveals curious behaviour - MDK2 isn't failing to fade in, it's failing to draw the scene at all.

[sarah-walker-pcem]

Quite a curious bug in the end. MDK2 is executing an FSCALE instruction with input parameters of 0.0 and 1536.0, this should perform 0.0 * (2 ^ 1536.0), which is obviously zero. 2 ^ 1536.0 is out of the range of any floating point format, and therefore resolves to infinity. On x87, 0.0 * infinity = NaN, which then propagates itself through numerous subsequent instructions. However this only seems to apply to explicit multiplies, this does not seem to occur to the multiply in FSCALE, and the end result should instead be 0.0. Adding special case code for FSCALE fixes MDK2.

[sarah-walker-pcem]

Fixed in commit 2e037b5.