When developing an OS for the smallest Atmels (ATtiny861/461/21) i observed a strange effect. Sometimes, depending on the optimization choosen by gcc, the compiler generates code which cannot be correct (in my view). Most of the time the code is not functional too.
The problem is that use of the instructions "std Y+1,rX" and "ldd rX,Y+1" is made, but that de corresponding registers are never properly initialized. The result is that the value is stored somewhere in RAM or is retrieved from that (random?) place. That may or may not hurt, but it seems wrong anyhow. Below you can see an piece of code generated by GCC (v4.2.3) 78c: 99 83 std Y+1, r25 ; 0x01 78e: 46 de rcall .-884 ; 0x41c <privOperateSlotStack> 790: 99 81 ldd r25, Y+1 ; 0x01 (From the methode below) The point is nowhere in that method the registers r28 and r29 are used neither are they set in the called methods. (see below) In practice this effect indeed results in non functional code which can most of the time be easily circumvented by inlining some methods or hooking some variables up to fixed registers. GCC is then forced to alter the output somewhat and often the error disappears. BTW, it seems that the compiler actually meant to do something like this: 78c: 79 2e mov r7, r25 ; 0x01 78e: 46 de rcall .-884 ; 0x41c <privOperateSlotStack> 790: 97 2d mov r25, r7 ; 0x01 where r7 can be any free register, since if you substitute that (in the raw hex file for example) the code turns fully functional. I have observed the effect also at GCC 4.2.1 and GCC 4.3.0, although beit not with exactly the same code. The effect is reproducible, but depends critically on the choose optimizations and structure of the code. These are my compiler options avr-gcc -c -save-temps -mmcu=attiny861 -mint8 -Wall -Wno-main \ -Winline -Wundef -gdwarf-2 -Os -funsigned-char -fomit-frame-pointer \ -fpack-struct -fshort-enums OK, it would take to much space to include enough details here for you to be able to reproduce the problem. I made a zip which can be downloaded from: http://www.femtoos.org/gcc_effect.zip which should make this possible. (includes source code, and a compiler script, you need avr-gcc 4.2.3 on your system, other versions will no reproduce the error in this particular example) regards, Ruud 0000074e <privQueuRequestBody>: 74e: 08 2f mov r16, r24 750: c6 2e mov r12, r22 752: 7a 01 movw r14, r20 754: 0d de rcall .-998 ; 0x370 <privInitOs> 756: fb dd rcall .-1034 ; 0x34e <tcbCurrent> 758: 5c 01 movw r10, r24 75a: 6c 2d mov r22, r12 75c: 80 2f mov r24, r16 75e: 09 dd rcall .-1518 ; 0x172 <privQueuTest> 760: 98 2f mov r25, r24 762: 1c 14 cp r1, r12 764: 24 f4 brge .+8 ; 0x76e <privQueuRequestBody+0x20> 766: 8c 15 cp r24, r12 768: 08 f4 brcc .+2 ; 0x76c <privQueuRequestBody+0x1e> 76a: 57 c0 rjmp .+174 ; 0x81a <privQueuRequestBody+0xcc> 76c: 07 c0 rjmp .+14 ; 0x77c <privQueuRequestBody+0x2e> 76e: cc 20 and r12, r12 770: 29 f0 breq .+10 ; 0x77c <privQueuRequestBody+0x2e> 772: 8c 2d mov r24, r12 774: 81 95 neg r24 776: 98 17 cp r25, r24 778: 08 f4 brcc .+2 ; 0x77c <privQueuRequestBody+0x2e> 77a: 4f c0 rjmp .+158 ; 0x81a <privQueuRequestBody+0xcc> 77c: 90 e0 ldi r25, 0x00 ; 0 77e: 10 2f mov r17, r16 780: 1f 70 andi r17, 0x0F ; 15 782: f0 e8 ldi r31, 0x80 ; 128 784: df 2e mov r13, r31 786: d1 2a or r13, r17 788: 6d 2d mov r22, r13 78a: 89 2f mov r24, r25 78c: 99 83 std Y+1, r25 ; 0x01 78e: 46 de rcall .-884 ; 0x41c <privOperateSlotStack> 790: 99 81 ldd r25, Y+1 ; 0x01 792: 88 23 and r24, r24 794: 31 f0 breq .+12 ; 0x7a2 <privQueuRequestBody+0x54> 796: 07 ff sbrs r16, 7 798: 40 c0 rjmp .+128 ; 0x81a <privQueuRequestBody+0xcc> 79a: 60 2f mov r22, r16 79c: 89 2f mov r24, r25 79e: 26 de rcall .-948 ; 0x3ec <privReleaseTask> 7a0: 3c c0 rjmp .+120 ; 0x81a <privQueuRequestBody+0xcc> 7a2: 9f 5f subi r25, 0xFF ; 255 7a4: 93 30 cpi r25, 0x03 ; 3 7a6: 09 f4 brne .+2 ; 0x7aa <privQueuRequestBody+0x5c> 7a8: 77 c0 rjmp .+238 ; 0x898 <privQueuRequestBody+0x14a> 7aa: ee cf rjmp .-36 ; 0x788 <privQueuRequestBody+0x3a> 7ac: f6 01 movw r30, r12 7ae: 81 81 ldd r24, Z+1 ; 0x01 7b0: 8f 7d andi r24, 0xDF ; 223 7b2: 81 83 std Z+1, r24 ; 0x01 7b4: e1 14 cp r14, r1 7b6: f1 04 cpc r15, r1 7b8: 49 f1 breq .+82 ; 0x80c <privQueuRequestBody+0xbe> 7ba: bd dd rcall .-1158 ; 0x336 <privCheckOsStack> 7bc: c8 dd rcall .-1136 ; 0x34e <tcbCurrent> 7be: 8c 01 movw r16, r24 7c0: f0 e0 ldi r31, 0x00 ; 0 7c2: ef 16 cp r14, r31 7c4: ff ef ldi r31, 0xFF ; 255 7c6: ff 06 cpc r15, r31 7c8: 18 f0 brcs .+6 ; 0x7d0 <privQueuRequestBody+0x82> 7ca: 60 e0 ldi r22, 0x00 ; 0 7cc: 8f ec ldi r24, 0xCF ; 207 7ce: 0a dd rcall .-1516 ; 0x1e4 <privShowError> 7d0: 80 91 90 00 lds r24, 0x0090 7d4: 20 91 8f 00 lds r18, 0x008F 7d8: 99 27 eor r25, r25 7da: 98 2f mov r25, r24 7dc: 88 27 eor r24, r24 7de: 82 0f add r24, r18 7e0: 91 1d adc r25, r1 7e2: 8e 0d add r24, r14 7e4: 9f 1d adc r25, r15 7e6: f8 01 movw r30, r16 7e8: 82 83 std Z+2, r24 ; 0x02 7ea: 89 2f mov r24, r25 7ec: 99 27 eor r25, r25 7ee: 83 83 std Z+3, r24 ; 0x03 7f0: 93 81 ldd r25, Z+3 ; 0x03 7f2: 80 91 90 00 lds r24, 0x0090 7f6: 98 17 cp r25, r24 7f8: 29 f4 brne .+10 ; 0x804 <privQueuRequestBody+0xb6> 7fa: 80 91 b1 00 lds r24, 0x00B1 7fe: 80 62 ori r24, 0x20 ; 32 800: 80 93 b1 00 sts 0x00B1, r24 804: f8 01 movw r30, r16 806: 81 81 ldd r24, Z+1 ; 0x01 808: 8f 7e andi r24, 0xEF ; 239 80a: 81 83 std Z+1, r24 ; 0x01 80c: f5 01 movw r30, r10 80e: 84 81 ldd r24, Z+4 ; 0x04 810: 8c 7f andi r24, 0xFC ; 252 812: 81 60 ori r24, 0x01 ; 1 814: 84 83 std Z+4, r24 ; 0x04 816: 80 e0 ldi r24, 0x00 ; 0 818: 3d c0 rjmp .+122 ; 0x894 <privQueuRequestBody+0x146> 81a: 00 68 ori r16, 0x80 ; 128 81c: 98 dd rcall .-1232 ; 0x34e <tcbCurrent> 81e: 4c 01 movw r8, r24 820: 0f 70 andi r16, 0x0F ; 15 822: 00 61 ori r16, 0x10 ; 16 824: 80 91 b1 00 lds r24, 0x00B1 828: 60 2f mov r22, r16 82a: 8f 70 andi r24, 0x0F ; 15 82c: f7 dd rcall .-1042 ; 0x41c <privOperateSlotStack> 82e: f4 01 movw r30, r8 830: 81 81 ldd r24, Z+1 ; 0x01 832: 8f 7d andi r24, 0xDF ; 223 834: 81 83 std Z+1, r24 ; 0x01 836: e1 14 cp r14, r1 838: f1 04 cpc r15, r1 83a: 49 f1 breq .+82 ; 0x88e <privQueuRequestBody+0x140> 83c: 7c dd rcall .-1288 ; 0x336 <privCheckOsStack> 83e: 87 dd rcall .-1266 ; 0x34e <tcbCurrent> 840: 8c 01 movw r16, r24 842: f0 e0 ldi r31, 0x00 ; 0 844: ef 16 cp r14, r31 846: ff ef ldi r31, 0xFF ; 255 848: ff 06 cpc r15, r31 84a: 18 f0 brcs .+6 ; 0x852 <privQueuRequestBody+0x104> 84c: 60 e0 ldi r22, 0x00 ; 0 84e: 8f ec ldi r24, 0xCF ; 207 850: c9 dc rcall .-1646 ; 0x1e4 <privShowError> 852: 80 91 90 00 lds r24, 0x0090 856: 20 91 8f 00 lds r18, 0x008F 85a: 99 27 eor r25, r25 85c: 98 2f mov r25, r24 85e: 88 27 eor r24, r24 860: 82 0f add r24, r18 862: 91 1d adc r25, r1 864: 8e 0d add r24, r14 866: 9f 1d adc r25, r15 868: f8 01 movw r30, r16 86a: 82 83 std Z+2, r24 ; 0x02 86c: 89 2f mov r24, r25 86e: 99 27 eor r25, r25 870: 83 83 std Z+3, r24 ; 0x03 872: 93 81 ldd r25, Z+3 ; 0x03 874: 80 91 90 00 lds r24, 0x0090 878: 98 17 cp r25, r24 87a: 29 f4 brne .+10 ; 0x886 <privQueuRequestBody+0x138> 87c: 80 91 b1 00 lds r24, 0x00B1 880: 80 62 ori r24, 0x20 ; 32 882: 80 93 b1 00 sts 0x00B1, r24 886: f8 01 movw r30, r16 888: 81 81 ldd r24, Z+1 ; 0x01 88a: 8f 7e andi r24, 0xEF ; 239 88c: 81 83 std Z+1, r24 ; 0x01 88e: f5 01 movw r30, r10 890: c5 82 std Z+5, r12 ; 0x05 892: 81 e0 ldi r24, 0x01 ; 1 894: e0 dd rcall .-1088 ; 0x456 <privEnterOS> 896: 0b c0 rjmp .+22 ; 0x8ae <portInit> 898: 5a dd rcall .-1356 ; 0x34e <tcbCurrent> 89a: 6c 01 movw r12, r24 89c: 10 61 ori r17, 0x10 ; 16 89e: 80 91 b1 00 lds r24, 0x00B1 8a2: 61 2f mov r22, r17 8a4: 8f 70 andi r24, 0x0F ; 15 8a6: ba dd rcall .-1164 ; 0x41c <privOperateSlotStack> 8a8: 07 ff sbrs r16, 7 8aa: b0 cf rjmp .-160 ; 0x80c <privQueuRequestBody+0xbe> 8ac: 7f cf rjmp .-258 ; 0x7ac <privQueuRequestBody+0x5e> -- Summary: Compiler makes use of [std Y+1, rX] instruction without initializing r28,r29 Product: gcc Version: 4.2.3 Status: UNCONFIRMED Severity: major Priority: P3 Component: target AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: ruud at betaresearch dot nl GCC build triplet: i686-linux-gentoo GCC host triplet: i686-linux-gentoo GCC target triplet: avr-*-* http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35807
