gcc 2.95.3/ARM problems comparing with negative
Toralf Lund
toralf@procaptura.com
Mon Nov 1 09:52:00 GMT 2004
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...
Help, anyone?
- Toralf
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