efriedma added a comment.
Consider something like this:
define i32 @div(i32 %x, i32 %y) {
entry:
%div = sdiv i32 %x, %y
ret i32 %div
}
We can mark this function readnone, but not speculatable: it doesn't read or
write memory, but could exhibit undefined behavior.
Consider another function:
@x = common local_unnamed_addr global i32 0, align 4
define i32 @get_x() {
entry:
%0 = load i32, i32* @x, align 4, !tbaa !2
ret i32 %0
}
We can mark this function speculatable, but not readnone: it doesn't exhibit
undefined behavior or have side-effects, but it does read memory.
Given that, it seems a little dubious to translate `__attribute__((const))`
into `speculatable`.
https://reviews.llvm.org/D33774
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