As of rev ea222be0d, LLVMs assembler will actually try to honour the
"fill value" part of p2align directives. X86 printed these as 0x90, which
isn't actually what it wanted: we want multi-byte nops for .text
padding. Compiling via a textual assembly file produces single-byte
nop padding since ea222be0d but the built-in assembler will produce
multi-byte nops. This divergent behaviour is undesirable.
To fix: don't set the byte padding field for x86, which allows the
assembler to pick multi-byte nops. Test that we get the same multi-byte
padding when compiled via textual assembly or directly to object file.
Added same-align-bytes-with-llasm-llobj.ll to that effect, updated
numerous other tests to not contain check-lines for the explicit padding.
basic block section cases
MachineBlockPlacement pass sets an alignment attribute to the loop
header MBB and this attribute will lead to an alignment directive during
emitting asm. In the case of the basic block section, the alignment
directive is put before the section label, and thus the alignment is set
to the predecessor of the loop header, which is not what we expect and
increases the code size (both inserting nop and set section alignment).
Reviewed By: rahmanl
Differential Revision: https://reviews.llvm.org/D137535
