gcc vs. indentation
The story begun when i`ve started to make indentation fixes in the DAC960 driver.
And in order to ensure i didn`t broke anything i was checking a diff between the resulting object files.
Surprisingly enough i`ve realised soon that indeed some indentation changes
give gcc a reason to produce different code.
One of the cases is below, all three of them are in the attached .tar.gz file. The code in question is the 2.5.72-bk1 kernel, however there was no changes
in the related code for some time, so plain .72 should be safe.
The examples are in the form of pairs of a C diff, and a "objdump -d" output diff.
The C diff:
drivers/block/DAC960.c 2003-06-29 22:11:01.000000000 +0400 renamed from drivers/block/DAC960.c 2003-06-17 01:09:50.000000000 +0400 +1 -2
@@ -272,8 +272,7 @@ dma_addr_t RequestSenseDMA; struct pci_pool *RequestSensePool = NULL; - if (Controller->FirmwareType == DAC960_V1_Controller) - { + if (Controller->FirmwareType == DAC960_V1_Controller) { CommandAllocationLength = offsetof(DAC960_Command_T, V1.EndMarker); CommandAllocationGroupSize = DAC960_V1_CommandAllocationGroupSize; ScatterGatherPool = pci_pool_create("DAC960_V1_ScatterGather",
newDAC960.o.d 2003-06-29 22:13:46.000000000 +0400 renamed from origDAC960.o.d 2003-06-29 21:02:55.000000000 +0400 +4 -4
@@ -1,5 +1,5 @@ -origDAC960.o: file format elf32-i386 +./newDAC960.o: file format elf32-i386 Disassembly of section .text:
@@ -5837,7 +5837,7 @@ 52a8: 84 c0 test %al,%al 52aa: 75 14 jne 52c0 <DAC960_V1_ProcessCompletedCommand+0x80> 52ac: 0f 0b ud2a - 52ae: 7d 0d jge 52bd <DAC960_V1_ProcessCompletedCommand+0x7d> + 52ae: 7c 0d jl 52bd <DAC960_V1_ProcessCompletedCommand+0x7d> 52b0: 27 daa 52b1: 00 00 add %al,(%eax) 52b3: 00 8d b6 00 00 00 add %cl,0xb6(%ebp)
@@ -5951,7 +5951,7 @@ 5421: 84 c0 test %al,%al 5423: 0f 85 97 fe ff ff jne 52c0 <DAC960_V1_ProcessCompletedCommand+0x80> 5429: 0f 0b ud2a - 542b: 8f 0d 27 00 00 00 popl 0x27 + 542b: 8e 0d 27 00 00 00 movl 0x27,%cs 5431: e9 8a fe ff ff jmp 52c0 <DAC960_V1_ProcessCompletedCommand+0x80> 5436: 89 1c 24 mov %ebx,(%esp,1) 5439: e8 fc ff ff ff call 543a <DAC960_V1_ProcessCompletedCommand+0x1fa>
@@ -7414,7 +7414,7 @@ 6ba2: 84 c0 test %al,%al 6ba4: 75 0a jne 6bb0 <DAC960_V2_ProcessCompletedCommand+0xa0> 6ba6: 0f 0b ud2a - 6ba8: bc 11 27 00 00 mov $0x2711,%esp + 6ba8: bb 11 27 00 00 mov $0x2711,%ebx 6bad: 00 89 f6 83 bc 24 add %cl,0x24bc83f6(%ecx) 6bb3: 84 00 test %al,(%eax) 6bb5: 00 00 add %al,(%eax)
Thats it.
The point is i thought and hoped that gcc abstract syntax tree constructor is indentation invariant, and that is seemingly not true.
regards, Samium Gromoff
Quoted 38 lines · Samium Gromoff
newDAC960.o.d 2003-06-29 22:13:46.000000000 +0400 renamed from origDAC960.o.d 2003-06-29 21:02:55.000000000 +0400 +5
-6- if (Controller->FirmwareType == DAC960_V1_Controller) - { + if (Controller->FirmwareType == DAC960_V1_Controller) { -origDAC960.o: file format elf32-i386 +./newDAC960.o: file format elf32-i386 Disassembly of section .text:@@ -5837,7 +5837,7 @@ 52a8: 84 c0 test %al,%al 52aa: 75 14 jne 52c0 <DAC960_V1_ProcessCompletedCommand+0x80> 52ac: 0f 0b ud2a - 52ae: 7d 0d jge 52bd <DAC960_V1_ProcessCompletedCommand+0x7d> + 52ae: 7c 0d jl 52bd <DAC960_V1_ProcessCompletedCommand+0x7d> 52b0: 27 daa 52b1: 00 00 add %al,(%eax) 52b3: 00 8d b6 00 00 00 add %cl,0xb6(%ebp)@@ -5951,7 +5951,7 @@ 5421: 84 c0 test %al,%al 5423: 0f 85 97 fe ff ff jne 52c0 <DAC960_V1_ProcessCompletedCommand+0x80> 5429: 0f 0b ud2a - 542b: 8f 0d 27 00 00 00 popl 0x27 + 542b: 8e 0d 27 00 00 00 movl 0x27,%cs 5431: e9 8a fe ff ff jmp 52c0 <DAC960_V1_ProcessCompletedCommand+0x80> 5436: 89 1c 24 mov %ebx,(%esp,1) 5439: e8 fc ff ff ff call 543a <DAC960_V1_ProcessCompletedCommand+0x1fa>@@ -7414,7 +7414,7 @@ 6ba2: 84 c0 test %al,%al 6ba4: 75 0a jne 6bb0 <DAC960_V2_ProcessCompletedCommand+0xa0> 6ba6: 0f 0b ud2a - 6ba8: bc 11 27 00 00 mov $0x2711,%esp + 6ba8: bb 11 27 00 00 mov $0x2711,%ebx 6bad: 00 89 f6 83 bc 24 add %cl,0x24bc83f6(%ecx) 6bb3: 84 00 test %al,(%eax) 6bb5: 00 00 add %al,(%eax)
Quoted 3 lines
Thats it.
The point is i thought and hoped that gcc abstract syntax tree constructor is indentation invariant, and that is seemingly not true.
It's okay, no need to worry. See the "ud2a"s just above the differences? Those are BUG()s, and they're going to be followed by a short __LINE__ then __FILE__ pointer. Your indentation change removed one line, so the BUG()'s __LINE__ numbers have gone down one. (And it takes a while for the disassembly to get back to sanity with the instructions thereafter.)
Hugh
Quoted 39 lines · On Mon, 30 Jun 2003 08:04:03 +0100 (BST) Hugh Dickins <[email protected]> wrote:
Quoted 38 lines · Samium Gromoff
newDAC960.o.d 2003-06-29 22:13:46.000000000 +0400 renamed from origDAC960.o.d 2003-06-29 21:02:55.000000000 +0400 +5
-6- if (Controller->FirmwareType == DAC960_V1_Controller) - { + if (Controller->FirmwareType == DAC960_V1_Controller) { -origDAC960.o: file format elf32-i386 +./newDAC960.o: file format elf32-i386 Disassembly of section .text:@@ -5837,7 +5837,7 @@ 52a8: 84 c0 test %al,%al 52aa: 75 14 jne 52c0 <DAC960_V1_ProcessCompletedCommand+0x80> 52ac: 0f 0b ud2a - 52ae: 7d 0d jge 52bd <DAC960_V1_ProcessCompletedCommand+0x7d> + 52ae: 7c 0d jl 52bd <DAC960_V1_ProcessCompletedCommand+0x7d> 52b0: 27 daa 52b1: 00 00 add %al,(%eax) 52b3: 00 8d b6 00 00 00 add %cl,0xb6(%ebp)@@ -5951,7 +5951,7 @@ 5421: 84 c0 test %al,%al 5423: 0f 85 97 fe ff ff jne 52c0 <DAC960_V1_ProcessCompletedCommand+0x80> 5429: 0f 0b ud2a - 542b: 8f 0d 27 00 00 00 popl 0x27 + 542b: 8e 0d 27 00 00 00 movl 0x27,%cs 5431: e9 8a fe ff ff jmp 52c0 <DAC960_V1_ProcessCompletedCommand+0x80> 5436: 89 1c 24 mov %ebx,(%esp,1) 5439: e8 fc ff ff ff call 543a <DAC960_V1_ProcessCompletedCommand+0x1fa>@@ -7414,7 +7414,7 @@ 6ba2: 84 c0 test %al,%al 6ba4: 75 0a jne 6bb0 <DAC960_V2_ProcessCompletedCommand+0xa0> 6ba6: 0f 0b ud2a - 6ba8: bc 11 27 00 00 mov $0x2711,%esp + 6ba8: bb 11 27 00 00 mov $0x2711,%ebx 6bad: 00 89 f6 83 bc 24 add %cl,0x24bc83f6(%ecx) 6bb3: 84 00 test %al,(%eax) 6bb5: 00 00 add %al,(%eax)
Quoted 9 lines
Quoted 3 lines
Thats it.
The point is i thought and hoped that gcc abstract syntax tree constructor is indentation invariant, and that is seemingly not true.It's okay, no need to worry. See the "ud2a"s just above the differences? Those are BUG()s, and they're going to be followed by a short
__LINE__then__FILE__pointer. Your indentation change removed one line, so the BUG()'s__LINE__numbers have gone down one. (And it takes a while for the disassembly to get back to sanity with the instructions thereafter.)
Uhhuh, i see now... those by-one differences looked strange for me... :-)
Quoted 4 lines
Hugh
Signature · 1 line
regards, Samium Gromoff
Samium Gromoff <[email protected]> said:
[...]
Quoted 2 lines
Surprisingly enough i`ve realised soon that indeed some indentation changes
give gcc a reason to produce different code.
[...]
Quoted 4 lines
newDAC960.o.d 2003-06-29 22:13:46.000000000 +0400 renamed from origDAC960.o.d 2003-06-29 21:02:55.000000000 +0400 +1
-1- 52ae: 7d 0d jge 52bd <DAC960_V1_ProcessCompletedCommand+0x7d> + 52ae: 7c 0d jl 52bd <DAC960_V1_ProcessCompletedCommand+0x7d>
[...]
Quoted 2 lines
newDAC960.o.d 2003-06-29 22:13:46.000000000 +0400 renamed from origDAC960.o.d 2003-06-29 21:02:55.000000000 +0400 +1
-1- 542b: 8f 0d 27 00 00 00 popl 0x27 + 542b: 8e 0d 27 00 00 00 movl 0x27,%cs
[...]
newDAC960.o.d 2003-06-29 22:13:46.000000000 +0400 line 7417
Quoted 2 lines
newDAC960.o.d 2003-06-29 22:13:46.000000000 +0400 renamed from origDAC960.o.d 2003-06-29 21:02:55.000000000 +0400 +1
-1- 6ba8: bc 11 27 00 00 mov $0x2711,%esp + 6ba8: bb 11 27 00 00 mov $0x2711,%ebx
This looks like 1-bit errors to my eye...
Signature · 4 lines
Dr. Horst H. von Brand User #22616 counter.li.org Departamento de Informatica Fono: +56 32 654431 Universidad Tecnica Federico Santa Maria +56 32 654239 Casilla 110-V, Valparaiso, Chile Fax: +56 32 797513