[Bug gas/29145] New: GAS always generates padding instructions regardless of `--no-pad-sections`

[lh_mouse at 126 dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29145

            Bug ID: 29145
           Summary: GAS always generates padding instructions regardless
                    of `--no-pad-sections`
           Product: binutils
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gas
          Assignee: unassigned at sourceware dot org
          Reporter: lh_mouse at 126 dot com
  Target Milestone: ---

Given this simple C program:

```
int foo(int a);
int bar(int a) { return foo(a);  }
```

We compile it with GCC, targeting linux:

```
lh_mouse@lhmouse-dev ~ $ x86_64-linux-gnu-gcc -Os test.c -S -o test.s && cat
test.s
        .file   "test.c"
        .text
        .globl  bar
        .type   bar, @function
bar:
.LFB0:
        .cfi_startproc
        jmp     foo@PLT
        .cfi_endproc
.LFE0:
        .size   bar, .-bar
        .ident  "GCC: (Debian 8.3.0-6) 8.3.0"
        .section        .note.GNU-stack,"",@progbits
```

The function contains only a `jmp` instruction. We assemble this file:

```
lh_mouse@lhmouse-dev ~ $ x86_64-linux-gnu-as test.s -o test.o && objdump -h
test.o

test.o:     file format elf64-x86-64

Sections:
Idx Name          Size      VMA               LMA               File off  Algn
  0 .text         00000005  0000000000000000  0000000000000000  00000040  2**0
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  1 .data         00000000  0000000000000000  0000000000000000  00000045  2**0
                  CONTENTS, ALLOC, LOAD, DATA
  2 .bss          00000000  0000000000000000  0000000000000000  00000045  2**0
                  ALLOC
  3 .comment      0000001d  0000000000000000  0000000000000000  00000045  2**0
                  CONTENTS, READONLY
  4 .note.GNU-stack 00000000  0000000000000000  0000000000000000  00000062 
2**0
                  CONTENTS, READONLY
  5 .eh_frame     00000030  0000000000000000  0000000000000000  00000068  2**3
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
```

The `.text` section has a size of 5 and alignment of 1, which looks good, and
does what `-Os` is presumed to do.


We attempt to compile the same code targeting mingw-w64:

```
lh_mouse@lhmouse-dev ~ $ x86_64-w64-mingw32-gcc -Os test.c -S -o test.s && cat
test.s
        .file   "test.c"
        .text
        .globl  bar
        .def    bar;    .scl    2;      .type   32;     .endef
        .seh_proc       bar
bar:
        .seh_endprologue
        jmp     foo
        .seh_endproc
        .ident  "GCC: (GNU) 8.3-win32 20190406"
        .def    foo;    .scl    2;      .type   32;     .endef
```

The function still contains only a `jmp` instruction. But when we assemble it:

```
lh_mouse@lhmouse-dev ~ $ x86_64-w64-mingw32-as test.s -o test.o && objdump -h
test.o

test.o:     file format pe-x86-64

Sections:
Idx Name          Size      VMA               LMA               File off  Algn
  0 .text         00000010  0000000000000000  0000000000000000  00000104  2**4
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  1 .data         00000000  0000000000000000  0000000000000000  00000000  2**4
                  ALLOC, LOAD, DATA
  2 .bss          00000000  0000000000000000  0000000000000000  00000000  2**4
                  ALLOC
  3 .xdata        00000004  0000000000000000  0000000000000000  00000114  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  4 .pdata        0000000c  0000000000000000  0000000000000000  00000118  2**2
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
  5 .rdata$zzz    00000020  0000000000000000  0000000000000000  00000124  2**4
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
```

This time the `.text` section has a size of 16 and alignment of 16 bytes, which
is undesired.


This also results in extra NOPs in the object file:

```
lh_mouse@lhmouse-dev ~ $ objdump -d test.o

test.o:     file format pe-x86-64


Disassembly of section .text:

0000000000000000 <bar>:
   0:   e9 00 00 00 00          jmpq   5 <bar+0x5>
   5:   90                      nop
   6:   90                      nop
   7:   90                      nop
   8:   90                      nop
   9:   90                      nop
   a:   90                      nop
   b:   90                      nop
   c:   90                      nop
   d:   90                      nop
   e:   90                      nop
   f:   90                      nop
```

which LD will not be able to remove.

Adding `.p2align` directives in 'test.s' seems only able to increase the
alignment (requests for 32, 64, etc. work), but it is not possible to decrease
the alignment (requests for 1, 2, 4, 8 are ignroed). Could this be improved a
little?

-- 
You are receiving this mail because:
You are on the CC list for the bug.