[LLVMdev] Isel DAG documentation? (original) (raw)

David Given dg at cowlark.com
Tue Mar 11 04:36:40 PDT 2014

On 3/9/14, 10:40 PM, David Given wrote:

That seems sensible... but I'm afraid it doesn't work, dying in precisely the same way as above.

So I think I've reached the bottom of this.

It looks like the generated table loses type information. At one point I had a rule that looked like this:

def MOVr : S16< (outs GR32:$rD), (ins GR32:$rS), "mov rD,rD, rD,rS", [(set GR32:$rD, GR32:$rS)]

;

This ended up last on the opcode list, and was always invoked if no other instruction matched --- because the selection pattern is basically 'x = y' from the point of view of the instruction selector, and therefore fits everything.

I believe that the same thing's applying to the globaladdr issue. I had rules that look like this:

def MOVar : S32<_ _(outs GR32:$rD), (ins i32imm:$addr), // i32imm is wrong here_ _"mov rD,rD, rD,addr", [(set GR32:$rD, tglobaladdr:$addr)]

; def : Pat<(globaladdr:$addr), (MOVar tglobaladdr:$addr)>;

...but that (set) ends up as a rule which compiles to 'x = y' and everything goes horribly wrong.

What I eventually did was to copy what the ARM target does: add code to TargetLowering::LowerOperation() to explicitly convert the globaladdr nodes into wrapped machine-specific nodes. My MOVar now looks like:

def MOVar : S32< (outs GR32:$rD), (ins i32imm:$addr), // i32imm is wrong here "mov rD,rD, rD,addr", [(set GR32:$rD, (WrappedGlobal tglobaladdr:$addr))]

;

The WrappedGlobal has no semantic meaning, simply being a machine-specific ISD node with a single parameter. I'm not quite sure why this works; it is, after all, the failing rule above should do precisely the same thing, but in a less cryptic way.

I suspect there's an llvm bug here, but I'm not quite sure what it is. It would be nice if tablegen was a bit more rigorous about detecting invalid patterns (is there an option I'm missing?).

-- ┌─── ｄｇ＠ｃｏｗｌａｒｋ．ｃｏｍ ───── http://www.cowlark.com ───── │ │ "You cannot truly appreciate Atlas Shrugged until you have read it │ in the original Klingon." --- Sea Wasp on r.a.sf.w

-------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 876 bytes Desc: OpenPGP digital signature URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140311/4fd6f34e/attachment.sig>

More information about the llvm-dev mailing list