[PATCH] phi-opt: Add missed optimization for "(cond | (a != b)) ? b : a"

[Jovan Vukic]

Currently, within the phiopt pass, under value_replacement, we have the
option to replace the expression "(cond & (a == b)) ? a : b" with "b". It
checks whether there is a BIT_AND_EXPR and verifies if one of the operands
contains the expression "a == b".

However, the same result should also apply to the expression with the
inverted condition "(cond | (a != b)) ? b : a," so this patch makes slight
modifications to the existing code to cover this situation as well. The
patch adds a check for whether there is a BIT_IOR_EXPR and if it contains
"a != b".

I added a test for both optimizations, as I could not find an adequate test
in the testsuite for the first one (even though that optimization exists).

For example, for this C code:

int foo(int a, int b, int c) {
    return ((a > c) | (a != b)) ? b : a;
}

Before the patch, the generated assembly for RISC-V is:

foo:
        bgt     a0, a2, .L4
        bne     a0, a1, .L4
        ret
.L4:
        mv      a0, a1
        ret

After the patch, the generated assembly becomes:

foo:
        mv      a0, a1
        ret

2024-10-22  Jovan Vukic  <Jovan.Vukic@rt-rk.com>

gcc/ChangeLog:

        * tree-ssa-phiopt.cc
        (rhs_is_fed_for_value_replacement): Add a new optimization opportunity
        for BIT_IOR_EXPR and a != b.
        (operand_equal_for_value_replacement): Ditto.

gcc/testsuite/ChangeLog:

        * gcc.dg/tree-ssa/phi-opt-same-3.c: New test.


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