[PATCH] D65143: [InstCombine] Fold '(-1 u/ %x) u< %y' to '@llvm.umul.with.overflow' + overflow bit extraction
Roman Lebedev via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Tue Jul 23 07:00:46 PDT 2019
lebedev.ri created this revision.
lebedev.ri added reviewers: nikic, spatel, efriedma, xbolva00, RKSimon.
lebedev.ri added a project: LLVM.
Herald added a subscriber: hiraditya.
lebedev.ri added a child revision: D65144: [InstCombine] Fold '((%x * %y) u/ %x) != %y' to '@llvm.umul.with.overflow' + overflow bit extraction.
`(-1 u/ %x) u< %y` is one of (3?) common ways to check that
some unsigned multiplication (will not) overflow.
Currently, we don't catch it. We could:
----------------------------------------
Name: no overflow
%o0 = udiv i4 -1, %x
%r = icmp ult i4 %o0, %y
=>
%o0 = udiv i4 -1, %x
%n0 = umul_overflow i4 %x, %y
%r = extractvalue {i4, i1} %n0, 1
Done: 1
Optimization is correct!
----------------------------------------
Name: no overflow, swapped
%o0 = udiv i4 -1, %x
%r = icmp ugt i4 %y, %o0
=>
%o0 = udiv i4 -1, %x
%n0 = umul_overflow i4 %x, %y
%r = extractvalue {i4, i1} %n0, 1
Done: 1
Optimization is correct!
----------------------------------------
Name: overflow
%o0 = udiv i4 -1, %x
%r = icmp uge i4 %o0, %y
=>
%o0 = udiv i4 -1, %x
%n0 = umul_overflow i4 %x, %y
%n1 = extractvalue {i4, i1} %n0, 1
%r = xor %n1, -1
Done: 1
Optimization is correct!
----------------------------------------
Name: overflow
%o0 = udiv i4 -1, %x
%r = icmp ule i4 %y, %o0
=>
%o0 = udiv i4 -1, %x
%n0 = umul_overflow i4 %x, %y
%n1 = extractvalue {i4, i1} %n0, 1
%r = xor %n1, -1
Done: 1
Optimization is correct!
As it can be observed from tests, while simply forming the `@llvm.umul.with.overflow`
is easy, if we were looking for the inverted answer, then more work needs to be done
to cleanup the now-pointless control-flow that was guarding against division-by-zero.
This is being addressed in follow-up patches.
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D65143
Files:
llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
llvm/test/Transforms/InstCombine/unsigned-mul-lack-of-overflow-check-via-udiv-of-allones.ll
llvm/test/Transforms/InstCombine/unsigned-mul-overflow-check-via-udiv-of-allones.ll
llvm/test/Transforms/PhaseOrdering/unsigned-multiply-overflow-check.ll
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