Re: [Qemu-devel] [PATCH] Fix overflow conditions for MIPS add/subtract

[Fabrice Bellard <fabrice@bellard.org>]

The current code seems correct to me too (it is the same as the x86 
"reference").

Fabrice.

Stefan Weil wrote:
> Dirk Behme schrieb:
> 
>  >
>  > Fix overflow conditions for MIPS add/subtract as proposed by
>  > Daniel Jacobowitz.
>  >
>  > http://lists.gnu.org/archive/html/qemu-devel/2006-04/msg00538.html
>  >
>  > Regards
>  >
>  > Dirk
>  >
>  >------------------------------------------------------------------------
>  >
>  >--- target-mips/op.c_orig 2006-04-30 09:40:46.000000000 +0200
>  >+++ target-mips/op.c 2006-04-30 09:41:52.000000000 +0200
>  >@@ -206,7 +206,7 @@ void op_addo (void)
>  >
>  > tmp = T0;
>  > T0 += T1;
>  >- if (((tmp ^ T1 ^ (-1)) & (T0 ^ T1)) >> 31) {
>  >+ if (~(T0 ^ T1) & (T0 ^ tmp) & 0x80000000) {
>  > /* operands of same sign, result different sign */
>  > CALL_FROM_TB1(do_raise_exception_direct, EXCP_OVERFLOW);
>  > }
>  >@@ -225,7 +225,7 @@ void op_subo (void)
>  >
>  > tmp = T0;
>  > T0 = (int32_t)T0 - (int32_t)T1;
>  >- if (((tmp ^ T1) & (tmp ^ T0)) >> 31) {
>  >+ if ((T0 ^ T1) & (T0 ^ tmp) & 0x80000000) {
>  > /* operands of different sign, first operand and result different 
> sign */
>  > CALL_FROM_TB1(do_raise_exception_direct, EXCP_OVERFLOW);
>  > }
>  >
>  >
>  >
>  >
>  >------------------------------------------------------------------------
>  >
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>  >
>  >
> 
> Hello Dirk,
> 
> which additions / subtractions are handled incorrectly by the current code?
> Here is the result of a test which shows that the current code (which is
> based on my patch)
> raises an exception for 0x80000000 + 0x80000000.
> 
> Daniel, perhaps you could sent the code you used to check overflow
> conditions?
> Maybe there is no need to change functions op_addo and op_subo for MIPS.
> 
> Regards
> Stefan
> 
> IN:
> 0x94000000: lui v0,0x8000
> 0x94000004: lui v1,0x8000
> 0x94000008: add a0,v0,v1
> 0x9400000c: b 0x9400000c
> 0x94000010: nop
> 
> ---------------- 2 00000002
> do_raise_exception_err: 19 0
> do_interrupt enter: PC 94000008 EPC 00000000 cause -1 excp 19
> do_interrupt: PC bfc00380 EPC 94000008 cause 12 excp 19
> S 10400000 C 00000030 A 00000000 D 00000000
> cpu_mips_handle_mmu_fault pc bfc00380 ad bfc00380 rw 2 is_user 0 smmu 1
> cpu_mips_handle_mmu_fault address=bfc00380 ret 0 physical 1fc00380 prot 1
> ------------------------------------------------
> pc=0xbfc00380 HI=0x00000000 LO=0x00000000 ds 0006 00000000 0
> GPR00: r0 00000000 at 00000000 v0 80000000 v1 80000000
> GPR04: a0 00000000 a1 00000000 a2 00000000 a3 00000000
> GPR08: t0 00000000 t1 00000000 t2 00000000 t3 00000000
> GPR12: t4 00000000 t5 00000000 t6 00000000 t7 00000000
> GPR16: s0 00000000 s1 00000000 s2 00000000 s3 00000000
> GPR20: s4 00000000 s5 00000000 s6 00000000 s7 00000000
> GPR24: t8 00000000 t9 00000000 k0 00000000 k1 00000000
> GPR28: gp 00000000 sp 94001040 s8 00000000 ra 00000000
> CP0 Status 0x10400006 Cause 0x00000030 EPC 0x94000008
> Config0 0x80008090 Config1 0x1e9b4d8a LLAddr 0x00000000
> 
> 
> 
> 
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