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[PATCH] Fix PR72517, improve vector extract RTL expansion

The following patch fixes a bug in selecting a "better" mode to
do a vector extract from.  The existing code was evidently written
without knowing what GET_MODE_WIDER_MODE does for vector modes
in the face of multiple supported vector sizes.  This results in
odd code-generation choices for HImode extract from V16QI for

The patch adds a missing constraint - make the new vector element
mode have the same size as the extraction mode as this is what
vec_extract_optab requires.

With the patch the testcase gcc.dg/vect/slp-45.c doesn't spill anymore
with -msse4.1 or -mavx2 while it spills plenty without it.

Bootstrap and regtest ongoing on x86_64-unknown-linux-gnu.

Note that the patch will now happily choose to extract DImode
punning the source to V4DImode from V8SFmode (because the expander
appearantly always chooses an integer mode for bit extraction).
Hopefully we don't have targets that have separate register files
for float vs. int vectors (or a heavy penalty in such punning).
Note this is a pre-existing issue as well - we just less likely
spill now (which possibly may made have such targets happy).


2016-07-26  Richard Biener  <>

	PR middle-end/72517
	* expmed.c (extract_bit_field_1): Constrain the vector mode
	with element size matching the extraction mode size when
	choosing a better vector mode to do the extraction from.

Index: gcc/expmed.c
*** gcc/expmed.c	(revision 238743)
--- gcc/expmed.c	(working copy)
*************** extract_bit_field_1 (rtx str_rtx, unsign
*** 1581,1586 ****
--- 1581,1587 ----
        for (; new_mode != VOIDmode ; new_mode = GET_MODE_WIDER_MODE (new_mode))
  	if (GET_MODE_SIZE (new_mode) == GET_MODE_SIZE (GET_MODE (op0))
+ 	    && GET_MODE_UNIT_SIZE (new_mode) == GET_MODE_SIZE (tmode)
  	    && targetm.vector_mode_supported_p (new_mode))
        if (new_mode != VOIDmode)

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