yeah,
you're right ...
overflow is unusable here.
will fix it in a minute.
~d
On Tuesday 15 July 2003 03:51, Takahashi, Chris wrote:
> I have discovered code which optimizes improperly. Attached are multiple
> versions of the code. The code is incorrect if any level of optimization
> is on (-O1 -O2 -O3 or -Os). test.c is compileable under gcc and
> msp430-gcc. When targeted at i386 the source generates valid code with both
> -D_WORK_ (the _WORK_ define switches from a valid to invalid version when
> compiled) and with out (in fact it creates the same code). Also attached
> is math.c which contains the problematic code with minimal support code so
> that a minimal object can be created. math.c also is the same as the
> preprocessed code of math.c.
>
> When compiled for the MSP target it uses msp430 simulator to assess the
> validity of the code. When the dysfunctional code is compiled it creates a
> jl instruction in place of a jn instruction after the if statement (see
> attached math.lst).
>
> I have included a make file to make viewing the bug easier. All you need
> is the simulator in your path and it will make the code and run the test
> cases.
>
> About the code:
> The attached code (in math.c) increments the upper nibble of temp1 by 3
> then
>
> examines the upper nibble of temp1 to see if it is >= 8. If it is then
> do nothing, otherwise
> undo the increment by decrementing the upper nibble by 3.
>
> The assembly generated performs the inc by 3 then checks the n and v flags
> with the jl instruction to determine when to jump. because only the last
> bit is being checked it is only appropriate to check the n flag -- as the
> result of the last arithmetic instr is inconsequential (aside from the
> content of bit 7).
>
> If any more info is needed please mail me or the list. And I'll send it
> ASAP.
>
> -Chris Takahashi
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