On 10/14/23 16:14, Roger Sayle wrote:
This patch is my proposed solution to PR rtl-optimization/91865.
Normally RTX simplification canonicalizes a ZERO_EXTEND of a ZERO_EXTEND
to a single ZERO_EXTEND, but as shown in this PR it is possible for
combine's make_compound_operation to unintentionally generate a
non-canonical ZERO_EXTEND of a ZERO_EXTEND, which is unlikely to be
matched by the backend.
For the new test case:
const int table[2] = {1, 2};
int foo (char i) { return table[i]; }
compiling with -O2 -mlarge on msp430 we currently see:
Trying 2 -> 7:
2: r25:HI=zero_extend(R12:QI)
REG_DEAD R12:QI
7: r28:PSI=sign_extend(r25:HI)#0
REG_DEAD r25:HI
Failed to match this instruction:
(set (reg:PSI 28 [ iD.1772 ])
(zero_extend:PSI (zero_extend:HI (reg:QI 12 R12 [ iD.1772 ]))))
which results in the following code:
foo: AND #0xff, R12
RLAM.A #4, R12 { RRAM.A #4, R12
RLAM.A #1, R12
MOVX.W table(R12), R12
RETA
With this patch, we now see:
Trying 2 -> 7:
2: r25:HI=zero_extend(R12:QI)
REG_DEAD R12:QI
7: r28:PSI=sign_extend(r25:HI)#0
REG_DEAD r25:HI
Successfully matched this instruction:
(set (reg:PSI 28 [ iD.1772 ])
(zero_extend:PSI (reg:QI 12 R12 [ iD.1772 ])))
allowing combination of insns 2 and 7
original costs 4 + 8 = 12
replacement cost 8
foo: MOV.B R12, R12
RLAM.A #1, R12
MOVX.W table(R12), R12
RETA
This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures. Ok for mainline?
2023-10-14 Roger Sayle <ro...@nextmovesoftware.com>
gcc/ChangeLog
PR rtl-optimization/91865
* combine.cc (make_compound_operation): Avoid creating a
ZERO_EXTEND of a ZERO_EXTEND.
gcc/testsuite/ChangeLog
PR rtl-optimization/91865
* gcc.target/msp430/pr91865.c: New test case.
Neither an ACK or NAK at this point.
The bug report includes a patch from Segher which purports to fix this
in simplify-rtx. Any thoughts on Segher's approach and whether or not
it should be considered?
The BZ also indicates that removal of 2 patterns from msp430.md would
solve this too (though it may cause regressions elsewhere?). Any
thoughts on that approach as well?
Thanks,
jeff
