Tool/software: TI C/C++ Compiler
When using optimizer levels 2 and above (O2, O3, O4) then the code for the strnlen_wtf() shown below gets incorrectly optimized to always return zero. For the similar but slightly different strnlen_ok() function this does not happen. Why?
Version info:
- Compiler v18.12.5LTS (and v18.12.4LTS)
- CCS: 9.3
- OS: Windows 10 1909 64bit
Here is the code.c example (fully self contained, no includes required):
typedef unsigned long size_t; // taken from stdlib.h to avoid code bloat in example
// this gets optimized away for O2, O3, O4. (OK for O0, O1):
size_t strnlen_wtf( const char *s, size_t maxlen )
{
size_t n = (size_t)-1;
do n++; while ( (*s++) && (n < maxlen) );
return n;
}
// this works:
size_t strnlen_ok( char const* s, size_t maxlen )
{
size_t n = 0;
do
{
} while ( (*(s++)) && ++n < maxlen );
return n;
}
When checking the object code with dis2000 we get:
/*
Compiled with O2 or O3 or O4: strnlen_wtf() gets incorrectly optimized to always return 0
"C:\\ti\\ccs930\\ccs\\tools\\compiler\\ti-cgt-c2000_18.12.5.LTS\\bin\\cl2000" -v28 -ml -mt --cla_support=cla1 --float_support=fpu32 --tmu_support=tmu0 --vcu_support=vcu2 -O2 --opt_for_speed=0 --include_path=W:/compiler_error_optimized_away --include_path=C:/ti/ccs930/ccs/tools/compiler/ti-cgt-c2000_18.12.5.LTS/include -g --diag_warning=225 --diag_wrap=off --display_error_number --abi=coffabi "..\\code.c"
"C:\\ti\\ccs930\\ccs\\tools\\compiler\\ti-cgt-c2000_18.12.5.LTS\\bin\\cl2000" -v28 -ml -mt --cla_support=cla1 --float_support=fpu32 --tmu_support=tmu0 --vcu_support=vcu2 -O3 --opt_for_speed=0 --include_path=W:/compiler_error_optimized_away --include_path=C:/ti/ccs930/ccs/tools/compiler/ti-cgt-c2000_18.12.5.LTS/include -g --diag_warning=225 --diag_wrap=off --display_error_number --abi=coffabi "..\\code.c"
"C:\\ti\\ccs930\\ccs\\tools\\compiler\\ti-cgt-c2000_18.12.5.LTS\\bin\\cl2000" -v28 -ml -mt --cla_support=cla1 --float_support=fpu32 --tmu_support=tmu0 --vcu_support=vcu2 -O4 --opt_for_speed=0 --include_path=W:/compiler_error_optimized_away --include_path=C:/ti/ccs930/ccs/tools/compiler/ti-cgt-c2000_18.12.5.LTS/include -g --diag_warning=225 --diag_wrap=off --display_error_number --abi=coffabi "..\\code.c"
Disassembly of code.obj:
TEXT Section .text (Little Endian), 0x14 words at 0x00000000
00000000 _strnlen_wtf:
00000000 0200 MOVB ACC, #0
00000001 0006 LRETR
00000002 _strnlen_ok:
00000002 ff58 TEST ACC
00000003 6003 SB 3, NEQ
00000004 d501 MOVB XAR5, #0x1
00000005 6f02 SB 2, UNC
00000006 $C$L1:
00000006 83a9 MOVL XAR5, ACC
00000007 $C$L2:
00000007 dd81 SUBB XAR5, #1
00000008 be00 MOVB XAR6, #0x00
00000009 $C$L3:
00000009 92c4 MOV AL, *+XAR4[0]
0000000a 6108 SB 8, EQ
0000000b 0200 MOVB ACC, #0
0000000c dd81 SUBB XAR5, #1
0000000d dc01 ADDB XAR4, #1
0000000e de01 ADDB XAR6, #1
0000000f 1901 SUBB ACC, #1
00000010 0fa5 CMPL ACC, XAR5
00000011 60f8 SB -8, NEQ
00000012 $C$L4:
00000012 06a6 MOVL ACC, XAR6
00000013 0006 LRETR
*/
/*
Compiled with O1 (OK):
""C:\\ti\\ccs930\\ccs\\tools\\compiler\\ti-cgt-c2000_18.12.5.LTS\\bin\\cl2000" -v28 -ml -mt --cla_support=cla1 --float_support=fpu32 --tmu_support=tmu0 --vcu_support=vcu2 -O1 --opt_for_speed=0 --include_path=W:/compiler_error_optimized_away --include_path=C:/ti/ccs930/ccs/tools/compiler/ti-cgt-c2000_18.12.5.LTS/include -g --diag_warning=225 --diag_wrap=off --display_error_number --abi=coffabi "..\\code.c"
Disassembly of code.obj:
TEXT Section .text (Little Endian), 0x16 words at 0x00000000
00000000 _strnlen_wtf:
00000000 1ea7 MOVL XAR7, ACC
00000001 0200 MOVB ACC, #0
00000002 1901 SUBB ACC, #1
00000003 1ea6 MOVL XAR6, ACC
00000004 $C$L1:
00000004 9284 MOV AL, *XAR4++
00000005 de01 ADDB XAR6, #1
00000006 6104 SB 4, EQ
00000007 06a7 MOVL ACC, XAR7
00000008 0fa6 CMPL ACC, XAR6
00000009 66fb SB -5, HI
0000000a $C$L2:
0000000a 06a6 MOVL ACC, XAR6
0000000b 0006 LRETR
0000000c _strnlen_ok:
0000000c d500 MOVB XAR5, #0x0
0000000d 1ea6 MOVL XAR6, ACC
0000000e $C$L3:
0000000e 9284 MOV AL, *XAR4++
0000000f 6105 SB 5, EQ
00000010 dd01 ADDB XAR5, #1
00000011 06a6 MOVL ACC, XAR6
00000012 0fa5 CMPL ACC, XAR5
00000013 66fb SB -5, HI
00000014 $C$L4:
00000014 a0a9 MOVL ACC, XAR5
00000015 0006 LRETR
*/
/*
Compiled with O0 (OK):
"C:\\ti\\ccs930\\ccs\\tools\\compiler\\ti-cgt-c2000_18.12.5.LTS\\bin\\cl2000" -v28 -ml -mt --cla_support=cla1 --float_support=fpu32 --tmu_support=tmu0 --vcu_support=vcu2 -O0 --opt_for_speed=0 --include_path=W:/compiler_error_optimized_away --include_path=C:/ti/ccs930/ccs/tools/compiler/ti-cgt-c2000_18.12.5.LTS/include -g --diag_warning=225 --diag_wrap=off --display_error_number --abi=coffabi "..\\code.c"
Disassembly of code.obj:
TEXT Section .text (Little Endian), 0x16 words at 0x00000000
00000000 _strnlen_wtf:
00000000 1ea7 MOVL XAR7, ACC
00000001 0200 MOVB ACC, #0
00000002 1901 SUBB ACC, #1
00000003 1ea6 MOVL XAR6, ACC
00000004 $C$L1:
00000004 de01 ADDB XAR6, #1
00000005 9284 MOV AL, *XAR4++
00000006 6104 SB 4, EQ
00000007 06a7 MOVL ACC, XAR7
00000008 0fa6 CMPL ACC, XAR6
00000009 66fb SB -5, HI
0000000a $C$L2:
0000000a 06a6 MOVL ACC, XAR6
0000000b 0006 LRETR
0000000c _strnlen_ok:
0000000c 1ea6 MOVL XAR6, ACC
0000000d d500 MOVB XAR5, #0x0
0000000e $C$L3:
0000000e 9284 MOV AL, *XAR4++
0000000f 6105 SB 5, EQ
00000010 dd01 ADDB XAR5, #1
00000011 06a6 MOVL ACC, XAR6
00000012 0fa5 CMPL ACC, XAR5
00000013 66fb SB -5, HI
00000014 $C$L4:
00000014 a0a9 MOVL ACC, XAR5
00000015 0006 LRETR
*/
