This pass detects cases of expensive store forwarding and tries to
avoid them by reordering the stores and using suitable bit insertion
sequences.  For example it can transform this:

     strb    w2, [x1, 1]
     ldr     x0, [x1]      # Expensive store forwarding to larger load.

To:

     ldr     x0, [x1]
     strb    w2, [x1]
     bfi     x0, x2, 0, 8

Assembly like this can appear with bitfields or type punning / unions.
On stress-ng when running the cpu-union microbenchmark the following
speedups have been observed.

  Neoverse-N1:      +29.4%
  Intel Coffeelake: +13.1%
  AMD 5950X:        +17.5%

The transformation is rejected on cases that cause store_bit_field to
generate subreg expressions on different register classes.  Files
avoid-store-forwarding-4.c and avoid-store-forwarding-5.c contain such
cases and have been marked as XFAIL.

Due to biasing of its operands in store_bit_field, there is a special
handling for machines with BITS_BIG_ENDIAN != BYTES_BIG_ENDIAN. The
need for this was exosed by an issue exposed on the H8 architecture,
which uses big-endian ordering, but BITS_BIG_ENDIAN is false. In that
case, the START parameter of store_bit_field needs to be calculated
from the end of the destination register.

gcc/ChangeLog:

	* Makefile.in (OBJS): Add avoid-store-forwarding.o.
	* common.opt (favoid-store-forwarding): New option.
	* common.opt.urls: Regenerate.
	* doc/invoke.texi: New param store-forwarding-max-distance.
	* doc/passes.texi: Document new pass.
	* doc/tm.texi: Regenerate.
	* doc/tm.texi.in: Document new pass.
	* params.opt (store-forwarding-max-distance): New param.
	* passes.def: Add pass_rtl_avoid_store_forwarding before
	pass_early_remat.
	* target.def (avoid_store_forwarding_p): New DEFHOOK.
	* target.h (struct store_fwd_info): Declare.
	* targhooks.cc (default_avoid_store_forwarding_p): New function.
	* targhooks.h (default_avoid_store_forwarding_p): Declare.
	* tree-pass.h (make_pass_rtl_avoid_store_forwarding): Declare.
	* avoid-store-forwarding.cc: New file.
	* avoid-store-forwarding.h: New file.
	* timevar.def (TV_AVOID_STORE_FORWARDING): New timevar.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/avoid-store-forwarding-1.c: New test.
	* gcc.target/aarch64/avoid-store-forwarding-2.c: New test.
	* gcc.target/aarch64/avoid-store-forwarding-3.c: New test.
	* gcc.target/aarch64/avoid-store-forwarding-4.c: New test.
	* gcc.target/aarch64/avoid-store-forwarding-5.c: New test.
	* gcc.target/x86_64/abi/callabi/avoid-store-forwarding-1.c: New test.
	* gcc.target/x86_64/abi/callabi/avoid-store-forwarding-2.c: New test.

Co-authored-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Signed-off-by: Konstantinos Eleftheriou <konstantinos.eleftheriou@vrull.eu>