[LLVMdev] Intrinsics and dead instruction/code elimination

[o.j.sivart at gmail.com]

On 20/05/2010, at 3:01 AM, Chris Lattner wrote:

> 
> On May 19, 2010, at 7:07 AM, o.j.sivart at gmail.com wrote:
> 
>> Hi all,
>> 
>> I'm interested in the impact of representing code via intrinsic functions, in contrast to via an instruction, when it comes to performing dead instruction/code elimination. As a concrete example, lets consider the simple case of the llvm.*.with.overflow.* intrinsics.
>> 
>> If I have some sequence (> 1) of llvm.*.with.overflow.* intrinsics, as in the form of:
>> 
>> @global = global i32 0
>> 
>> define void @fun() {
>> entry:
>> %res1 = call {i32, i1} @llvm.*.with.overflow.i32(i32 %a, i32 %b)
>> %sum1 = extractvalue {i32, i1} %res, 0
>> %obit1 = extractvalue {i32, i1} %res, 1
>> store i32 %obit1, i32* @global
>> ...
>> %res2 = call {i32, i1} @llvm.*.with.overflow.i32(i32 %a, i32 %b)
>> %sum2 = extractvalue {i32, i1} %res, 0
>> %obit2 = extractvalue {i32, i1} %res, 1
>> store i32 %obit2, i32* @global
>> }
>> 
>> then I assume an optimisation pass is able to eliminate the store i32 %obit1, i32* @global, since store i32 %obit2, i32* @global clearly clobbers the global without any interleaving load/access. However, my question is whether representing code as an intrinsic limits  further dead instruction/code elimination. In this case, the intrinsic will produce an arithmetic operation followed by a setcc on overflow on x86. Given the first store is dead, the first setcc on overflow instruction is also dead and can be eliminated. Is LLVM capable of such elimination with intrinsics, or is the expectation that an optimisation pass replace llvm.*.with.overflow.* intrinsics with their corresponding non-intrinsic arithmetics operations in order to achieve the same result, or is there another solution?
> 
> Intrinsics should be optimized as well as instructions.  In this specific case, these intrinsics should be marked readnone, which means that load/store optimization will ignore them.  Dead code elimination will delete the intrinsic if it is dead etc.

I understand that dead code elimination is able to delete the intrinsic if it is dead. What I'm interested in is whether or not, despite the entire intrinsic not being dead, anything is able to eliminate the setcc on overflow instruction part of the first intrinsic given that the store of obit1 is dead, thus obit1 is not needed, thus extracting the overflow bit from the CFLAGS register via a setcc instruction is no longer needed. I assume nothing is able to perform such optimisation on intrinsics and my guess is that the only option is, as I said, a pass which rewrites the first intrinsic to just its corresponding arithmetic instruction once a pass has dead store eliminated the first store. Is this the case?

> Are you seeing this fail on a specific testcase?

No I don't yet have code for the particular testcases I have in mind. I'm just exploring the limitations of intrinsics over instructions.

> -Chris