On 9/30/24 2:04 AM, Artemiy Volkov wrote:
Implement a match.pd transformation inverting the sign of X in
C1 - X cmp C2, where C1 and C2 are integer constants and X is
of a wrapping signed type, by observing that:
(a) If cmp is == or !=, simply move X and C2 to opposite sides of
the comparison to arrive at X cmp C1 - C2.
(b) If cmp is <:
- C1 - X < C2 means that C1 - X spans the values of -INF,
-INF + 1, ..., C2 - 1;
- Therefore, X is one of C1 - -INF, C1 - (-INF + 1), ...,
C1 - C2 + 1;
- Subtracting (C1 + 1), X - (C1 + 1) is one of - (-INF) - 1,
- (-INF) - 2, ..., -C2;
- Using the fact that - (-INF) - 1 is +INF, derive that
X - (C1 + 1) spans the values +INF, +INF - 1, ..., -C2;
- Thus, the original expression can be simplified to
X - (C1 + 1) > -C2 - 1.
(c) Similarly, C1 - X <= C2 is equivalent to X - (C1 + 1) >= -C2 - 1.
(d) The >= and > cases are negations of (b) and (c), respectively.
(e) In all cases, the expression -C2 - 1 can be shortened to
bit_not (C2).
This transformation allows to occasionally save load-immediate /
subtraction instructions, e.g. the following statement:
10 - (int)f() >= 20;
now compiles to
addi a0,a0,-11
slti a0,a0,-20
instead of
li a5,10
sub a0,a5,a0
slti t0,a0,20
xori a0,t0,1
on 32-bit RISC-V when compiled with -fwrapv.
Additional examples can be found in the newly added test file. This
patch has been bootstrapped and regtested on aarch64, x86_64, and i386,
and additionally regtested on riscv32.
gcc/ChangeLog:
PR tree-optimization/116024
* match.pd: New transformation around integer comparison.
gcc/testsuite/ChangeLog:
* gcc.dg/tree-ssa/pr116024-1-fwrapv.c: New test.
Thanks. I've pushed this to the trunk.
jeff
