[Bug middle-end/192] String literals don't obey -fdata-sections

[gcc at mattwhitlock dot name]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=192

--- Comment #11 from Matt Whitlock <gcc at mattwhitlock dot name> ---
Created attachment 35479
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35479&action=edit
put string literals into unique sections when -fmerge-constants -fdata-sections

This patch puts each string literal into a (probably) unique section when
compiling with -fmerge-constants -fdata-sections. The section name is
constructed from the character width and string alignment (as before) plus a
32-bit hash of the string contents.

Consider the following program:

  void used() {
      puts("keep me");
      puts("common");
      puts("string");
      puts("tail");
  }

  void not_used() {
      puts("toss me");
      puts("common");
      puts("ring");
      puts("entail");
  }

  int main() {
      used();
      return 0;
  }

$ gcc -ffunction-sections -fdata-sections -fmerge-constants \
      -Wl,--gc-sections -o test test.c

Compiling with an unpatched GCC produces a binary whose .rodata contains:

   40061d 6b656570 206d6500 636f6d6d 6f6e0073  keep me.common.s
   40062d 7472696e 6700746f 7373206d 6500656e  tring.toss me.en
   40063d 7461696c 00                          tail.           

Compiling with a patched GCC produces a binary whose .rodata contains:

   40061d 6b656570 206d6500 636f6d6d 6f6e0073  keep me.common.s
   40062d 7472696e 67007461 696c00             tring.tail.