This is exactly the problem - that's what I'd expect from the warning text, but
apparently, that is not what actually happens.
For "assume(a > 0 && i++)" the entire assumption gets thrown out.
void foo(int a, int b)
{
__builtin_assume(a > 0 && b++);
}
>clang -O0 -c -S -o - -emit-llvm assume.c
assume.c:3:20: warning: the argument to '__builtin_assume' has side effects
that will be discarded [-Wassume]
__builtin_assume(a > 0 && b++);
^~~~~~~~~~~~
; ModuleID = 'assume.c'
target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-windows-msvc"
; Function Attrs: nounwind uwtable
define void @foo(i32 %a, i32 %b) #0 {
entry:
%b.addr = alloca i32, align 4
%a.addr = alloca i32, align 4
store i32 %b, i32* %b.addr, align 4
store i32 %a, i32* %a.addr, align 4
ret void
}
-----Original Message-----
From: Aaron Ballman [mailto:[email protected]]
Sent: Monday, March 23, 2015 18:01
To: [email protected]
Cc: Kuperstein, Michael M; Hal Finkel; Richard Smith; llvm cfe
Subject: Re: [PATCH] Have HasSideEffects() return false for
__attribute__((const)) functions
On Mon, Mar 23, 2015 at 11:47 AM, Michael Kuperstein
<[email protected]> wrote:
> Also, there's something a bit odd about the warning text for an assume with
> side-effects.
>
> "the argument to '__assume' has side effects that will be discarded" suggests
> that only the side effects will be discarded.
Which is correct. The side effects are unevaluated, but __assume is not. Think:
sizeof(i++). i will not be incremented, but sizeof is still evaluated and
produces a result.
> This:
> (a) doesn't make a lot of sense to me, and
> (b) doesn't imply that nothing at all will be emitted.
> That is, for "assume(a > 0 && i++)", the warning would make me expect that
> what I ended up with was equivalent to "assume(a > 0)" or perhaps "assume(a >
> 0 && i)".
The latter is correct, not the former.
>
> Is this just a problem with how I'm reading the warning? If not, any
> suggestions for alternative text?
I'm not certain where the confusion lies, but if there is alternative text that
would be more clear, then great!
~Aaron
>
>
> http://reviews.llvm.org/D8548
>
> EMAIL PREFERENCES
> http://reviews.llvm.org/settings/panel/emailpreferences/
>
>
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