https://llvm.org/bugs/show_bug.cgi?id=28672
Bug ID: 28672
Summary: Integer divide by constant close to UINT*_MAX can be
compare rather than divide
Product: new-bugs
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: simon.ho...@arm.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
When doing unsigned division by an integer which is greater than `UINT_MAX / 2`
(or another range limit in place of UINT_MAX, as appropriate), the result can
be only 0 or 1, and this can be evaluated with a compare rather than a
reciprocal-multiply (or divide).
For example:
uint32_t f(uint32_t x) { return (x + 1) % 4000569407; }
is equivalent to:
uint32_t f(uint32_t x) { x++; return x < 4000569407 ? x : x - 4000569407; }
Clang gives me:
clang --target=arm-linux-gnueabihf -O3 -S -o- p1modk.c
ldr r1, .LCPI0_0 // 316062335
add r0, r0, #1
umull r1, r2, r0, r1
sub r1, r0, r2
add r1, r2, r1, lsr #1
ldr r2, .LCPI0_1 // 4000569407
lsr r1, r1, #31
mul r1, r1, r2
sub r0, r0, r1
I expect something more like:
ldr r1, =4000569407
add r0, r0, #1
cmp r0, r1
subhs r0, r0, r1
Note that this applicable in loops containing the `x = (x + 1) % k;` idiom if x
can be shown to be never greater than k (even if k is variable, I think).
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