[PATCH] x86_64: Improvements to arithmetic right shifts of V1TImode values.

[Roger Sayle]

This patch to the i386 backend's ix86_expand_v1ti_ashiftrt provides
improved (shorter) implementations of V1TI mode arithmetic right shifts
for constant amounts between 111 and 126 bits.  The significance of
this range is that this functionality is useful for (eventually)
providing sign extension from HImode and QImode to V1TImode.

For example, x>>112 (to sign extend a 16-bit value), was previously
generated as a four operation sequence:

        movdqa  %xmm0, %xmm1            // word    7 6 5 4 3 2 1 0
        psrad   $31, %xmm0              // V8HI = [S,S,?,?,?,?,?,?]
        psrad   $16, %xmm1              // V8HI = [S,X,?,?,?,?,?,?]
        punpckhqdq      %xmm0, %xmm1    // V8HI = [S,S,?,?,S,X,?,?]
        pshufd  $253, %xmm1, %xmm0      // V8HI = [S,S,S,S,S,S,S,X]

with this patch, we now generates a three operation sequence:

        psrad   $16, %xmm0              // V8HI = [S,X,?,?,?,?,?,?]
        pshufhw $254, %xmm0, %xmm0      // V8HI = [S,S,S,X,?,?,?,?]
        pshufd  $254, %xmm0, %xmm0      // V8HI = [S,S,S,S,S,S,S,X]

The correctness of generated code is confirmed by the existing
run-time test gcc.target/i386/sse2-v1ti-ashiftrt-1.c in the testsuite.
This idiom is safe to use for shifts by 127, but that case gets handled
by a two operation sequence earlier in this function.


This patch has been tested on x86_64-pc-linux-gnu with a make bootstrap
and make -k check with no new failures.  OK for mainline?


2022-01-11  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386-expand.c (ix86_expand_v1ti_ashiftrt): Provide
	new three operation implementations for shifts by 111..126 bits.


Thanks in advance,
Roger
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