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 an unsigned 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 range of 0, 1, ..., C2 - 1;
- This means that X spans the range of C1 - (C2 - 1),
C1 - (C2 - 2), ..., C1;
- Subtracting C1 - (C2 - 1), X - (C1 - (C2 - 1)) is one of 0, 1,
..., C1 - (C1 - (C2 - 1));
- Simplifying the above, X - (C1 - C2 + 1) is one of 0, 1, ...,
C2 - 1;
- Summarizing, the expression C1 - X < C2 can be transformed
into X - (C1 - C2 + 1) < C2.
(c) Similarly, if cmp is <=:
- C1 - X <= C2 means that C1 - X is one of 0, 1, ..., C2;
- It follows that X is one of C1 - C2, C1 - (C2 - 1), ..., C1;
- Subtracting C1 - C2, X - (C1 - C2) has range 0, 1, ..., C2;
- Thus, the expression C1 - X <= C2 can be transformed into
X - (C1 - C2) <= C2.
(d) The >= and > cases are negations of (b) and (c), respectively.
This transformation allows to occasionally save load-immediate /
subtraction instructions, e.g. the following statement:
300 - (unsigned int)f() < 100;
now compiles to
addi a0,a0,-201
sltiu a0,a0,100
instead of
li a5,300
sub a0,a5,a0
sltiu a0,a0,100
on 32-bit RISC-V.
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.c: New test.
Signed-off-by: Artemiy Volkov <arte...@synopsys.com>
Thanks. I've pushed this to the trunk.
jeff
