varasm: Ensure each variable in mergeable section is the entity size [PR121394]

Now there are mergeable sections which have an entity size, we can place
decls (constants) that are smaller in size in these sections.
An example is an `long double` which has a size of 12 bytes on i686 and is
placed in the 16 bytes shareable section.

For an example with the following C++ code:
```
std::initializer_list<long double> a = {0.3l};
```

We place the constant array in the .rodata.cst16 section but we don't add a padding to 16 bytes.
```
        .section        .rodata.cst16,"aM",@progbits,16
        .align 16
        .type   _ZGR1a_, @object
        .size   _ZGR1a_, 12
_ZGR1a_:
        .long   -1717986918
        .long   -1717986919
        .long   16381
        .text
```
GAS has a workaround added to do the padding but other assemblers don't.
The gas workaround was added with https://sourceware.org/legacy-ml/binutils/2002-11/msg00615.html .
Now for the constant pool, GCC does emit a `.align` to padd out the size correctly.
This was done in r0-46282-gf41115930523b3.
The same padding should be done when emitting a variable contents when in a mergeable section.

This patch implements the padding and we now get an addition `.zero 4` which pads out the section.

Bootstrapped and tested on x86_64-linux-gnu.

	PR middle-end/121394
gcc/ChangeLog:

	* varasm.cc (assemble_variable_contents): Pad out
	mergeable sections if needed.
	(output_constant_pool_1): Change the padding to be explicit
	zeroing for mergeable sections.

Signed-off-by: Andrew Pinski <[email protected]>

Author: apinski-quic

gcc/varasm.cc

@@ -2475,6 +2475,19 @@ assemble_variable_contents (tree decl, const char *name,
       else
 	/* Leave space for it.  */
 	assemble_zeros (tree_to_uhwi (DECL_SIZE_UNIT (decl)));
+      /* For mergeable section, make sure the section is zero filled up to
+	 the entity size of the section.  */
+      if (in_section
+	  && (in_section->common.flags & SECTION_MERGE)
+	  && tree_fits_uhwi_p (DECL_SIZE_UNIT (decl))
+	  && ((in_section->common.flags & SECTION_ENTSIZE)
+	      > tree_to_uhwi (DECL_SIZE_UNIT (decl))))
+	{
+	  unsigned HOST_WIDE_INT entsize, declsize;
+	  entsize = (in_section->common.flags & SECTION_ENTSIZE);
+	  declsize = tree_to_uhwi (DECL_SIZE_UNIT (decl));
+	  assemble_zeros (entsize - declsize);
+	}
       targetm.asm_out.decl_end ();
     }
 }
@@ -4435,10 +4448,16 @@ output_constant_pool_1 (class constant_descriptor_rtx *desc,
 
   /* Make sure all constants in SECTION_MERGE and not SECTION_STRINGS
      sections have proper size.  */
-  if (align > GET_MODE_BITSIZE (desc->mode)
-      && in_section
-      && (in_section->common.flags & SECTION_MERGE))
-    assemble_align (align);
+  if (in_section
+      && (in_section->common.flags & SECTION_MERGE)
+      && ((in_section->common.flags & SECTION_ENTSIZE)
+	   > GET_MODE_SIZE (desc->mode)))
+    {
+      unsigned HOST_WIDE_INT entsize, constsize;
+      entsize = (in_section->common.flags & SECTION_ENTSIZE);
+      constsize = GET_MODE_SIZE (desc->mode);
+      assemble_zeros (entsize - constsize);
+    }
 
 #ifdef ASM_OUTPUT_SPECIAL_POOL_ENTRY
  done: