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ARMCC: Bitwise Vs half word access

I was under the impression that the compiler would generate same code for read and write operation on the members of the following structure:

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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;
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

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:

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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;
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

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:

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$ 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
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

$ 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

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    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]
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    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:

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$ 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
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

$ 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

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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>
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

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)

Top replies

c0deface over 7 years ago in reply to Ronan Synnott +1 verified

Thanks a lot for clarification and my bad that I missed the ReadS16() and WriteS16() . Those are indeed the same as you have written there. That explains a lot the behavior and the implementation that...