C6000-CGT: Compiler bug with -O2 or higher

Tool/software:

Hello,

I encountered some unexpected behavior in one of our products and was able to track the behavior down to an apparent compiler bug. After some trial and error, I was able to work the code into a minimal test case, and I created a CCS project to demonstrate the bug. First, the compiler information:

• Compiler version: 8.3.13 (latest)
• Compiler options used: -mv6600 -O2 --include_path="C:/test/CompilerBugTest" --include_path="C:/test/ti-cgt-c6000_8.3.13/include" --preproc_with_comment --preproc_with_compile --diag_warning=225 --diag_wrap=off --display_error_number -k

The bug occurs when bugFunction() is compiled at optimization level 2 or higher. The assembler output shows a certain EXTU operation that does not make sense and ends up causing the bad behavior:

_Z11bugFunctionv:
...
           MVKL    .S2     testValues,B4     ; [B_Sb66] |15|
           MVKH    .S2     testValues,B4     ; [B_Sb66] |15|
           LDDW    .D2T2   *B4(0),B1:B0      ; [B_D64P] |15|
           NOP             4                 ; [A_L66]

           EXTU    .S2     B0,24,24,B0       ; [B_Sb66] |19| 
|| [!B1]   MVKL    .S1     outputStruct,A4   ; [A_S66] |21|
|| [!B1]   MVK     .L1     1,A3              ; [A_L66] |21|
...

The EXTU instruction zeroes out the upper 24 bits of B0, which is the register that contains the value testValues[0]. As a result, bugFunction() computes that both testValues[0] and testValues[1] are zero even when the upper 24 bits of testValues[0] are not zero.

The output of the test program is correct when bugFunction() is compiled at optimization level 1 or lower. The correct output is as follows:

testValuesAreBothZero: testValues[0] = 0x00000000, testValues[1] = 0x00000000

The incorrect output of the program occurs when bugFunction() is compiled at optimization level 2 or higher (only the final line of the output is actually correct):

testValuesAreBothZero: testValues[0] = 0x00000100, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00000200, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00000400, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00000800, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00001000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00002000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00004000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00008000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00010000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00020000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00040000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00080000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00100000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00200000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00400000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00800000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x01000000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x02000000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x04000000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x08000000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x10000000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x20000000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x40000000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x80000000, testValues[1] = 0x00000000
testValuesAreBothZero: testValues[0] = 0x00000000, testValues[1] = 0x00000000