https://llvm.org/bugs/show_bug.cgi?id=31844
Bug ID: 31844
Summary: Many small consecutive moves of constants aren't
optimized into something better.
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: george.burgess...@gmail.com
Reporter: george.burgess...@gmail.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Motivating example: https://godbolt.org/g/VOk6W6
Danny noted that test2 gets turned into good code because clang emits an
intermediate alloca to write the {0, 1, ...} array to, whereas in test1, {0, 1,
...} gets written directly to an sret param. As a result, SROA was able to work
on test2, but not test1.
The idea at the moment is that we should just make MemCpyOpt handle this; it's
happy to turn consecutive stores of the same value into a memset, but isn't
currently able to turn consecutive stores of different values into a memcpy:
http://llvm.org/docs/doxygen/html/MemCpyOptimizer_8cpp_source.html#l00626 .
I'm happy to see what I can do here, but this isn't at the top of my list. So,
if someone else wants to jump on this, feel free. :)
(Mildly verbose) repro: running `opt -O2` on the following code is effectively
a nop.
%struct.FooContainer = type { i16, %struct.Foo, i16 }
%struct.Foo = type { [16 x i8] }
; Function Attrs: norecurse nounwind uwtable
define void @_Z5test1v(%struct.FooContainer* noalias nocapture sret
%agg.result) local_unnamed_addr {
entry:
%a = getelementptr inbounds %struct.FooContainer, %struct.FooContainer*
%agg.result, i64 0, i32 0
store i16 0, i16* %a, align 2
%arrayinit.begin = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 0
store i8 0, i8* %arrayinit.begin, align 2
%arrayinit.element = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 1
store i8 1, i8* %arrayinit.element, align 1
%arrayinit.element2 = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 2
store i8 2, i8* %arrayinit.element2, align 2
%arrayinit.element3 = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 3
store i8 3, i8* %arrayinit.element3, align 1
%arrayinit.element4 = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 4
store i8 4, i8* %arrayinit.element4, align 2
%arrayinit.element5 = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 5
store i8 5, i8* %arrayinit.element5, align 1
%arrayinit.element6 = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 6
store i8 6, i8* %arrayinit.element6, align 2
%arrayinit.element7 = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 7
store i8 7, i8* %arrayinit.element7, align 1
%arrayinit.element8 = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 8
store i8 8, i8* %arrayinit.element8, align 2
%arrayinit.element9 = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 9
store i8 9, i8* %arrayinit.element9, align 1
%arrayinit.element10 = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 10
store i8 10, i8* %arrayinit.element10, align 2
%arrayinit.element11 = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 11
store i8 11, i8* %arrayinit.element11, align 1
%arrayinit.element12 = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 12
store i8 12, i8* %arrayinit.element12, align 2
%arrayinit.element13 = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 13
store i8 13, i8* %arrayinit.element13, align 1
%arrayinit.element14 = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 14
store i8 14, i8* %arrayinit.element14, align 2
%arrayinit.element15 = getelementptr inbounds %struct.FooContainer,
%struct.FooContainer* %agg.result, i64 0, i32 1, i32 0, i64 15
store i8 15, i8* %arrayinit.element15, align 1
%c = getelementptr inbounds %struct.FooContainer, %struct.FooContainer*
%agg.result, i64 0, i32 2
store i16 7, i16* %c, align 2
ret void
}
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