Subject: Re: [PATCH 00/22] add support for Clang LTO

From: Sami Tolvanen <[email-masked]>
Date: Tue, 7 Jul 2020 09:05:28 -0700

On Tue, Jul 07, 2020 at 08:51:07AM -0700, Sami Tolvanen wrote:
> After spending some time debugging this with Nick, it looks like the
> error is caused by a recent optimization change in LLVM, which together
> with the inlining of ur_load_imm_any into jeq_imm, changes a runtime
> check in FIELD_FIT that would always fail, to a compile-time check that
> breaks the build. In jeq_imm, we have:
> 
>     /* struct bpf_insn: _s32 imm */
>     u64 imm = insn->imm; /* sign extend */
>     ...
>     if (imm >> 32) { /* non-zero only if insn->imm is negative */
>     	/* inlined from ur_load_imm_any */
> 	u32 __imm = imm >> 32; /* therefore, always 0xffffffff */
> 
>         /*
> 	 * __imm has a value known at compile-time, which means
> 	 * __builtin_constant_p(__imm) is true and we end up with
> 	 * essentially this in __BF_FIELD_CHECK:
> 	 */
> 	if (__builtin_constant_p(__imm) && __imm <= 255)

Should be __imm > 255, of course, which means the compiler will generate
a call to __compiletime_assert.

> Jiong, Jakub, do you see any issues here?

(Jiong's email bounced, so removing from the recipient list.)

Sami

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