simplify-rtx: Extend (truncate (*extract ...)) fold [PR87763]

simplify-rtx: Extend (truncate (*extract ...)) fold [PR87763]

In the gcc.target/aarch64/lsl_asr_sbfiz.c part of this PR, we have:

Failed to match this instruction:
(set (reg:SI 95)
    (ashift:SI (subreg:SI (sign_extract:DI (subreg:DI (reg:SI 97) 0)
                (const_int 3 [0x3])
                (const_int 0 [0])) 0)
        (const_int 19 [0x13])))

If we perform the natural simplification to:

(set (reg:SI 95)
    (ashift:SI (sign_extract:SI (reg:SI 97)
                (const_int 3 [0x3])
                (const_int 0 [0])) 0)
        (const_int 19 [0x13])))

then the pattern matches.  And it turns out that we do have a
simplification like that already, but it would only kick in for
extractions from a reg, not a subreg.  E.g.:

(set (reg:SI 95)
    (ashift:SI (subreg:SI (sign_extract:DI (reg:DI X)
                (const_int 3 [0x3])
                (const_int 0 [0])) 0)
        (const_int 19 [0x13])))

would simplify to:

(set (reg:SI 95)
    (ashift:SI (sign_extract:SI (subreg:SI (reg:DI X) 0)
                (const_int 3 [0x3])
                (const_int 0 [0])) 0)
        (const_int 19 [0x13])))

IMO the subreg case is even more obviously a simplification
than the bare reg case, since the net effect is to remove
either one or two subregs, rather than simply change the
position of a subreg/truncation.

However, doing that regressed gcc.dg/tree-ssa/pr64910-2.c
for -m32 on x86_64-linux-gnu, because we could then simplify
a :HI zero_extract to a :QI one.  The associated *testqi_ext_3
pattern did already seem to want to handle QImode extractions:

  "ix86_match_ccmode (insn, CCNOmode)
   && ((TARGET_64BIT && GET_MODE (operands[2]) == DImode)
       || GET_MODE (operands[2]) == SImode
       || GET_MODE (operands[2]) == HImode
       || GET_MODE (operands[2]) == QImode)

but I'm not sure how often the QI case would trigger in practice,
since the zero_extract mode was restricted to HI and above.  I checked
the other x86 patterns and couldn't see any other instances of this.

2020-01-28  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR rtl-optimization/87763
	* simplify-rtx.c (simplify_truncation): Extend sign/zero_extract
	simplification to handle subregs as well as bare regs.
	* config/i386/i386.md (*testqi_ext_3): Match QI extracts too.