I was under the impression that the compiler would generate same code for read and write operation on the members of the following structure:
typedef struct S32data_t { uint32_t a:16; uint32_t b:16; } S32data_t; typedef struct S16data_t { uint16_t a; uint16_t b; } S16data_t;
The read and write operation that I'm performing are:
S32data_t test32data; S16data_t test16data; uint32_t xS32; uint32_t yS32; uint16_t xS16; uint16_t yS16; void WriteS32(void) { test32data.a = xS32; test32data.b = yS32; } int ReadS32(void) { xS32 = test32data.a; yS32 = test32data.b; return xS32; }
The corresponding assembly file generated by the compiler is as below:
$ armcc.exe --help Product: ARM Compiler 5.06 Component: ARM Compiler 5.06 update 6 (build 750) Tool: armcc [4d3637] $ armcc.exe -O3 --c99 -c test.c -o test.o; $ fromelf.exe -c test.o -o test.s
WriteS32 0x00000000: e59f00c0 .... LDR r0,[pc,#192] ; [0xc8] = 0 0x00000004: e1d020bc . .. LDRH r2,[r0,#0xc] 0x00000008: e2801004 .... ADD r1,r0,#4 0x0000000c: e1c120b0 . .. STRH r2,[r1,#0] 0x00000010: e1d001b0 .... LDRH r0,[r0,#0x10] 0x00000014: e1c100b2 .... STRH r0,[r1,#2] 0x00000018: e12fff1e ../. BX lr ReadS32 0x0000001c: e59f00a8 .... LDR r0,[pc,#168] ; [0xcc] = 0x4 0x00000020: e59f20a0 . .. LDR r2,[pc,#160] ; [0xc8] = 0 0x00000024: e5901000 .... LDR r1,[r0,#0] 0x00000028: e282200c . .. ADD r2,r2,#0xc 0x0000002c: e1a00801 .... LSL r0,r1,#16 0x00000030: e1a00820 ... LSR r0,r0,#16 0x00000034: e1a01821 !... LSR r1,r1,#16 0x00000038: e8820003 .... STM r2,{r0,r1} 0x0000003c: e12fff1e ../. BX lr WriteS16 0x00000040: e59f0080 .... LDR r0,[pc,#128] ; [0xc8] = 0 0x00000044: e1d020b0 . .. LDRH r2,[r0,#0] 0x00000048: e2801008 .... ADD r1,r0,#8 0x0000004c: e1c120b0 . .. STRH r2,[r1,#0] 0x00000050: e1d000b2 .... LDRH r0,[r0,#2] 0x00000054: e1c100b2 .... STRH r0,[r1,#2] 0x00000058: e12fff1e ../. BX lr ReadS16 0x0000005c: e59f106c l... LDR r1,[pc,#108] ; [0xd0] = 0x8 0x00000060: e1d100b0 .... LDRH r0,[r1,#0] 0x00000064: e2412008 . A. SUB r2,r1,#8 0x00000068: e1c200b0 .... STRH r0,[r2,#0] 0x0000006c: e1d110b2 .... LDRH r1,[r1,#2] 0x00000070: e1c210b2 .... STRH r1,[r2,#2] 0x00000074: e12fff1e ../. BX lr
If you'll notice, you'll see that the assembler has generated exactly the same code for Write operation but quite different for Read operation. I tried this same experiment with gcc on a x86_64 Linux machine and there I see that the assembler generated exactly the same code for both Write and Read operation. snip below:
$ uname -m x86_64 $ gcc --version gcc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609 $ gcc -c -O3 -g test.c -o test.o $ objdump -d test.c
0000000000000000 <WriteS32>: 0: 8b 05 00 00 00 00 mov 0x0(%rip),%eax # 6 <WriteS32+0x6> 6: 66 89 05 00 00 00 00 mov %ax,0x0(%rip) # d <WriteS32+0xd> d: 8b 05 00 00 00 00 mov 0x0(%rip),%eax # 13 <WriteS32+0x13> 13: 66 89 05 00 00 00 00 mov %ax,0x0(%rip) # 1a <WriteS32+0x1a> 1a: c3 retq 1b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 0000000000000020 <ReadS32>: 20: 0f b7 05 00 00 00 00 movzwl 0x0(%rip),%eax # 27 <ReadS32+0x7> 27: 0f b7 15 00 00 00 00 movzwl 0x0(%rip),%edx # 2e <ReadS32+0xe> 2e: 89 05 00 00 00 00 mov %eax,0x0(%rip) # 34 <ReadS32+0x14> 34: 89 15 00 00 00 00 mov %edx,0x0(%rip) # 3a <ReadS32+0x1a> 3a: c3 retq 3b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 0000000000000040 <WriteS16>: 40: 0f b7 05 00 00 00 00 movzwl 0x0(%rip),%eax # 47 <WriteS16+0x7> 47: 66 89 05 00 00 00 00 mov %ax,0x0(%rip) # 4e <WriteS16+0xe> 4e: 0f b7 05 00 00 00 00 movzwl 0x0(%rip),%eax # 55 <WriteS16+0x15> 55: 66 89 05 00 00 00 00 mov %ax,0x0(%rip) # 5c <WriteS16+0x1c> 5c: c3 retq 5d: 0f 1f 00 nopl (%rax) 0000000000000060 <ReadS16>: 60: 0f b7 05 00 00 00 00 movzwl 0x0(%rip),%eax # 67 <ReadS16+0x7> 67: 0f b7 15 00 00 00 00 movzwl 0x0(%rip),%edx # 6e <ReadS16+0xe> 6e: 66 89 05 00 00 00 00 mov %ax,0x0(%rip) # 75 <ReadS16+0x15> 75: 66 89 15 00 00 00 00 mov %dx,0x0(%rip) # 7c <ReadS16+0x1c> 7c: c3 retq 7d: 0f 1f 00 nopl (%rax)
Hence, Please let me know, why such behavior and Is it somehow possible to enforce the ARMCC compiler to generate exactly the same code for both cases (Bitwise and half word read and writes)