gcc 2.95.3/ARM problems comparing with negative
Richard Earnshaw
rearnsha@gcc.gnu.org
Mon Nov 1 10:20:00 GMT 2004
On Mon, 2004-11-01 at 09:52, Toralf Lund wrote:
> I'm having a rather strange problem with an ARM code built with gcc
> 2.95.3. Maybe I'm missing something obvious, but it looks like the wrong
> branch is taken after a comparison with a negative constant.
>
> I have the following test
>
> if(position<-ZSTP_MAX_POS)
>
>
> where ZSTP_MAX_POS is #defined as 250000, and "position" is an argument
> to the function containing the test. The problem is simply that if I
> pass e.g. 1 for the position argument, the test evaluates to "true" -
> according to the debugger, anyway - but surely 1 is not smaller than
> -250000? Also, the following code is generated for the test:
>
> 8013458: e51b3014 ldr r3, [fp, -#20]
> 801345c: e3e02a3d mvn r2, #249856 ; 0x3d000
> 8013460: e2422090 sub r2, r2, #144 ; 0x90
> 8013464: e1530002 cmp r3, r2
> 8013468: 8a000017 bhi 80134cc <_SetZoomPos+0xc4>
>
> This is followed by the "test true" code, i.e. as far as I can tell, the
> branch is supposed to be taken when the test is not true. But will it? I
> would expect "bgt" instead of "bhi" for the branch instruction, I think.
>
> All this with gcc 2.95.3. Maybe I could upgrade, but it seems to me that
> newer versions will actually generate less efficient code...
bhi is a branch used for an unsigned comparison. So I suspect your
problem is the type of one of your arguments, most likely the
declaration of position itself.
R.
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