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Re: [PATCH] Relax check against commuting XOR and ASHIFTRT in combine.c

Thanks for the suggestions! I think I've got a reasonably platform-independent testcase that scans the rtl dump, just trying it on a few more platforms now...

As to running on Alpha: bootstrap succeeds, and the regression testsuite doesn't raise any issues ( - and that's with a more aggressive patch that completely rolls back the original r76965:

Index: combine.c
--- combine.c   (revision 212523)
+++ combine.c   (working copy)
@@ -10218,9 +10218,6 @@
            if (CONST_INT_P (XEXP (varop, 1))
                /* We can't do this if we have (ashiftrt (xor))  and the
                   constant has its sign bit set in shift_mode.  */
-             && !(code == ASHIFTRT && GET_CODE (varop) == XOR
-                  && 0 > trunc_int_for_mode (INTVAL (XEXP (varop, 1)),
-                                             shift_mode))
                && (new_rtx = simplify_const_binary_operation
                    (code, result_mode,
                     gen_int_mode (INTVAL (XEXP (varop, 1)), result_mode),
@@ -10237,10 +10234,7 @@
               logical expression, make a new logical expression, and apply
               the inverse distributive law.  This also can't be done
               for some (ashiftrt (xor)).  */
-         if (CONST_INT_P (XEXP (varop, 1))
-            && !(code == ASHIFTRT && GET_CODE (varop) == XOR
-                 && 0 > trunc_int_for_mode (INTVAL (XEXP (varop, 1)),
-                                            shift_mode)))
+         if (CONST_INT_P (XEXP (varop, 1)))
                rtx lhs = simplify_shift_const (NULL_RTX, code, shift_mode,
                                                XEXP (varop, 0), count);

I'm testing this version more widely but initial indications are good.

However, I've not succeeded in checking Ada on Alpha, as GCC's Ada frontend requires an Ada compiler to bootstrap. So I have to ask: does anyone actually use Ada on Alpha? (And if so, would they please be able to test the above patch?)

Moreover, I don't really see we have much reason to believe the check against commuting is required even for Ada/Alpha. GCC's internals have changed substantially in the interim, with the Ada frontend no longer generating RTL directly, as we now have intervening GENERIC/GIMPLE tree stages. Unless there is a logical/bitwise explanation for why the commuting of ashiftrc and xor is unsafe, is the best explanation that the Ada frontend was generating RTL that may have looked OK at the time but we would now consider dubious, malformed, bad?

(E.g., these days I don't see how to produce an ashiftrt of one mode containing
an XOR of another without an intervening sign_extend, zero_extend or subreg.)


Jeff Law wrote:
On 06/30/14 13:05, Alan Lawrence wrote:
combine.c includes a check which prevents

(ashiftrt (xor A C2) C1)

from being commuted to

(xor (ashiftrt A C1) (ashiftrt C2 C1))

for constants C1, C2 if C2 has its sign bit set.

Specifically, this prevents (ashiftrt (not A) C1) from being commuted to

(not (ashiftrt A C1))

because the former is rewritten to (ashiftrt (xor A -1) C1) above, with
a comment /* Make this fit the case below.  */ - which it no longer does.

If result_mode == shift_mode, I can see no reason to prevent this
normalisation (indeed, I'm not sure I can see any reason to prevent it
even if result_mode != shift_mode - but I've not managed to produce such
a case in my own testing, as there are always intervening subreg's or
sign_extend's, or to build a toolchain on which to reproduce the
original bug, so I'm being cautious). Hence this patch allows
commutation if the two modes are equal.

As an illustrative example, on AArch64, without this patch, compiling
this snippet of the current arm_neon.h:

typedef long long int int64x1_t __attribute__ ((__vector_size__((8))));
int64x1 vcgez_s64(int64x1_t a)
   return (int64x1_t) {a >=0 ? -1 : 0};

produces a sequence involving a logical not (mvn) followed by asr (plus
some inter-bank moves), as the combiner takes (ashiftrt (not x) 63),
"simplifies", and fails to match the resulting RTL

(set (reg:DI 78 [ D.2593 ])
     (ashiftrt:DI (xor:DI (reg:DI 0 x0 [ aD.2565 ])
             (const_int -1 [0xffffffffffffffff]))
         (const_int 63 [0x3f])))

However, with this patch, the combiner simplifies to

(set (reg:DI 84 [ D.2604 ])
     (neg:DI (ge:DI (reg:DI 32 v0 [ aD.2569 ])
             (const_int 0 [0]))))

which matches a pattern to output the desired cmge instruction.

(For the curious: the check against commutation was added in January
2004, r76965, -
however, I've not been able to build an ADA toolchain of that era, or an
Alpha/VMS toolchain, on which to reproduce the bug; if anyone has such
and is willing and able to investigate, by all means!)


aarch64-none-elf: check-gcc (cross-tested)
x86-none-linux-gnu: bootstrapped; check-gcc, check-ada, check-g++
arm-none-linux-gnueabi: bootstrapped
arm-none-eabi: check-gcc (cross-tested)


     * combine.c (simplify_shift_const_1): Allow commuting ASHIFTRT and XOR
     if same mode.
You'll want to use your sample code from above to create a testcase. You can either dump the RTL and search it, or scan the assembly code.

Look in gcc/testsuite/ for examples.

Given the original problem which prompted Kenner to make this change was Ada related on the Alpha, you might ping rth and/or uros to see if they could do a "quick" regression of those platforms with Ada enabled.

I'm naturally hesitant to approve since this was something pretty Kenner explicitly tried to avoid AFAICT. Kenner wasn't ever known for trying to paper over problems -- if he checked in a change like this, much more likely than not it was well thought out and analyzed. He also probably knew combine.c better than anyone at that time (possibly even still true today).


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