Dynamic X86 CPU core dead locked with DYNFPU

Dynamic X86 CPU core dead locked with DYNFPU_DH enabled

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First post, by kjliew

Posted on 2014-03-12, 01:50

kjliew Offline

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Posts: 1304
Joined: 2004-01-08, 03:03

The game is Heavy Gear 2 under Win98SE, DOSBox SVN r3858 with voodoo1 patch. I also confirm that similar deadlock is also present in the game downloadable demo.
Here's the code snippet where it deadlocked.

1000A1063: 8BF9                           mov       edi,ecx
2000A1065: D1FF                           sar (d)   edi,1
3000A1067: 8BC7                           mov       eax,edi
4000A1069: C1E004                         shl (d)   eax,04
5000A106C: DD0418                         fld (q)   st(0),[ebx+eax]
6000A106F: DC5DD8                         fcomp (q) st(0),[ebp-28]
7000A1072: 8D3418                         lea       esi,[ebx+eax]
8000A1075: DFE0                           fstsw     ax
9000A1077: F6C441                         test (b)  ah,41
10000A107A: 741F                           je        file:000A109B
11000A107C: 8B06                           mov       eax,[esi]
12000A107E: C1E104                         shl (d)   ecx,04
13000A1081: 03CB                           add       ecx,ebx
14000A1083: 8901                           mov       [ecx],eax
15000A1085: 8B4604                         mov       eax,[esi+04]
16000A1088: 894104                         mov       [ecx+04],eax
17000A108B: 8B4608                         mov       eax,[esi+08]
18000A108E: 894108                         mov       [ecx+08],eax
19000A1091: 8B460C                         mov       eax,[esi+0C]
20000A1094: 89410C                         mov       [ecx+0C],eax
21000A1097: 8BCF                           mov       ecx,edi
22000A1099: EBC8                           jmps      file:000A1063

1st CacheBlock [A1063 - A1077]
2nd CacheBlock [A107C - A1097]
The jump instruction at A107A is never taken, so the code spins indefinitely. This jump closes the 1st CacheBlock as if it is taken, then it will link to another CacheBlock beyond A109B.
The deadlock is resolved when using "dynamic_nodhfpu" as CPU core, but FPU performance takes a significant hit. The slowdown is obviously noticeable in game.

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