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a49643a387
In LLVM IR the following code:
%r = urem <ty> %t, %b
is equivalent to
%q = udiv <ty> %t, %b
%s = mul <ty> nuw %q, %b
%r = sub <ty> nuw %t, %q ; (t / b) * b + (t % b) = t
As UDiv, Mul and Sub are already supported by SCEV, URem can be implemented
with minimal effort using that relation:
%r --> (-%b * (%t /u %b)) + %t
We implement two special cases:
- if %b is 1, the result is always 0
- if %b is a power-of-two, we produce a zext/trunc based expression instead
That is, the following code:
%r = urem i32 %t, 65536
Produces:
%r --> (zext i16 (trunc i32 %a to i16) to i32)
Note that while this helps get a tighter bound on the range analysis and the
known-bits analysis, this exposes some normalization shortcoming of SCEVs:
%div = udim i32 %a, 65536
%mul = mul i32 %div, 65536
%rem = urem i32 %a, 65536
%add = add i32 %mul, %rem
Will usually not be reduced.
llvm-svn: 312329
34 lines
730 B
LLVM
34 lines
730 B
LLVM
; RUN: opt < %s -scalar-evolution -analyze | FileCheck %s
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define i8 @foo(i8 %a) {
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; CHECK-LABEL: @foo
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%t0 = urem i8 %a, 27
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; CHECK: %t0 = urem i8 %a, 27
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; CHECK-NEXT: --> ((-27 * (%a /u 27)) + %a)
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ret i8 %t0
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}
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define i8 @bar(i8 %a) {
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; CHECK-LABEL: @bar
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%t1 = urem i8 %a, 1
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; CHECK: %t1 = urem i8 %a, 1
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; CHECK-NEXT: --> 0
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ret i8 %t1
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}
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define i8 @baz(i8 %a) {
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; CHECK-LABEL: @baz
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%t2 = urem i8 %a, 32
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; CHECK: %t2 = urem i8 %a, 32
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; CHECK-NEXT: --> (zext i5 (trunc i8 %a to i5) to i8)
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ret i8 %t2
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}
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define i8 @qux(i8 %a) {
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; CHECK-LABEL: @qux
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%t3 = urem i8 %a, 2
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; CHECK: %t3 = urem i8 %a, 2
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; CHECK-NEXT: --> (zext i1 (trunc i8 %a to i1) to i8)
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ret i8 %t3
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}
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