RE: ARM compiler bug?

"static unsigned int x = 0x80000000; // Very negative"

This is an unsigned number and is +2147483648(dec).

Have you verified that z is actually 0x7FFFFFFF ?

"Danny" wrote:

> Hi All,
>
> The following sample code gives me unexpected results.
>
> void FindBug()
> {
> static unsigned int x = 0x80000000; // Very negative
> static unsigned int y = 1; // Positive
> int z = x - y; // z = 0x7FFFFFFF
> if (z > 0) {
> OALMSG(TRUE, (L"z (0x%X) > 0\r\n", z));
> } else {
> OALMSG(TRUE, (L"z (0x%X) <= 0?\r\n", z));
> }
> }
>
> Looking at this code seems that I should get "z (0x7FFFFFFF) > 0", however,
> I actually get "z (0x7FFFFFFF) <= 0?".
> I have attached the generated assembler file below. I have embedded some
> comments in the assembler.
>
> 00000 |FindBug| PROC
>
> ; 37 : {
>
> 00000 |$L14233|
> 00000 e52de004 str lr, [sp, #-4]!
> 00004 |$M14231|
>
> ; 38 : static unsigned int x = 0x80000000; // Very negative
> ; 39 : static unsigned int y = 1; // Positive
> ; 40 : int z = x - y; // z = 0x7FFFFFFF
>
> 00004 e59f3024 ldr r3, [pc, #0x24]
> 00008 e5932004 ldr r2, [r3, #4]
> 0000c e5933000 ldr r3, [r3]
> 00010 e0521003 subs r1, r2, r3
>
> ; 41 : if (z > 0) {
> ; 42 : OALMSG(TRUE, (L"z (0x%X) > 0\r\n", z));
>
> -> At this point z = 0x7FFFFFFF (positive number).
>
> 00014 c59f0010 ldrgt r0, [pc, #0x10]
>
> -> We expect the ldrgt to execute but it does not (ldrle does).
> -> The condition for gt are Z = 0, N==V.
> -> Z = 0 We pass the first condition.
> -> N = 0 (bit 31) and V = 1 (overflow from negative to positive) We fail the
> second condition.
> -> Therefore, N != V and the ldrgt (>) does not execute, even though that is
> what we expect.
>
> -> Looks like a compiler bug.
>
> ; 43 : } else {
> ; 44 : OALMSG(TRUE, (L"z (0x%X) <= 0?\r\n", z));
>
> 00018 d59f0008 ldrle r0, [pc, #8]
> 0001c eb000000 bl NKDbgPrintfW
>
> ; 45 : }
> ; 46 : }
>
> 00020 e49de004 ldr lr, [sp], #4
> 00024 e12fff1e bx lr
> 00028 |$L14236|
> 00028 00000000 DCD
> |??_C@_1CC@EMHAJMPE@?$AAz?$AA?5?$AA?$CI?$AA0?$AAx?$AA?$CF?$AAX?$AA?$CJ?$AA?5
> ?$AA?$DM?$AA?$DN?$AA?5?$AA0?$AA?$DP?$AA?$AN?$AA?6?$AA?$AA@|
> 0002c 00000000 DCD
> |??_C@_1BO@FIKAIAKN@?$AAz?$AA?5?$AA?$CI?$AA0?$AAx?$AA?$CF?$AAX?$AA?$CJ?$AA?5
> ?$AA?$DO?$AA?5?$AA0?$AA?$AN?$AA?6?$AA?$AA@|
> 00030 00000000 DCD |?y@?1??FindBug@@9@9|
> 00034 |$M14232|
>
> ENDP ; |FindBug|
>
> Danny
>
>
>
.