Hello! I'm compiling code which uses pointers as iterators. For some
reason--probably a silly misunderstanding of the docs--I can't
eliminate duplicate pointer loads. I'll probably figure this out
eventually, but if somebody else sees the answer instantly, I
certainly won't complain. :-)
Here are the optimizers I'm running:
opt -f -simplifycfg -dce -instcombine -anders-aa -load-vn -licm -o
regex-opt.bc regex.bc
The duplicate loads appear at the top of the %regex6 and %regex2
blocks below. I've tried various alias analysis implementations
either alone or in combination.
Any thoughts? I'm sure that this will prove embarrassingly obvious. :-)
Cheers,
Eric
; ModuleID = 'regex-opt.bc'
implementation ; Functions:
bool %matches(ubyte* %begin, ubyte* %end) {
entry:
%scan_end = getelementptr ubyte* %end, int -1 ;
<ubyte*> [#uses=1]
br label %loop_test
regex6: ; preds = %loop_test
%c8 = load ubyte* %iter ; <ubyte> [#uses=1]
%matches9 = seteq ubyte %c8, 97 ; <bool> [#uses=1]
br bool %matches9, label %ret_true, label %regex2
regex2: ; preds = %regex6
%c = load ubyte* %iter ; <ubyte> [#uses=1]
%matches = seteq ubyte %c, 98 ; <bool> [#uses=1]
br bool %matches, label %ret_true, label %loop_step
loop_step: ; preds = %regex2
%next = getelementptr ubyte* %iter, int 1 ;
<ubyte*> [#uses=1]
br label %loop_test
loop_test: ; preds = %loop_step, %entry
%iter = phi ubyte* [ %begin, %entry ], [ %next, %
loop_step ] ; <ubyte*> [#uses=4]
%done = setgt ubyte* %iter, %scan_end ; <bool>
[#uses=1]
br bool %done, label %ret_false, label %regex6
ret_true: ; preds = %regex2, %regex6
ret bool true
ret_false: ; preds = %loop_test
ret bool false
}
On Thu, 16 Mar 2006, Eric Kidd wrote:> Hello! I'm compiling code which uses pointers as iterators. For some > reason--probably a silly misunderstanding of the docs--I can't eliminate > duplicate pointer loads. I'll probably figure this out eventually, but if > somebody else sees the answer instantly, I certainly won't complain. :-)There are no stupid questions. Some are just easier to answer than others :)> Here are the optimizers I'm running: > > opt -f -simplifycfg -dce -instcombine -anders-aa -load-vn -licm -o > regex-opt.bc regex.bc > > The duplicate loads appear at the top of the %regex6 and %regex2 blocks > below. I've tried various alias analysis implementations either alone or in > combination.LICM doesn't remove common subexpressions, also -load-vn doesn't affect LICM. Try "-licm -load-vn -gcse" instead of "-load-vn -licm" -Chris> Any thoughts? I'm sure that this will prove embarrassingly obvious. :-) > > Cheers, > Eric > > ; ModuleID = 'regex-opt.bc' > > implementation ; Functions: > > bool %matches(ubyte* %begin, ubyte* %end) { > entry: > %scan_end = getelementptr ubyte* %end, int -1 ; <ubyte*> > [#uses=1] > br label %loop_test > > regex6: ; preds = %loop_test > %c8 = load ubyte* %iter ; <ubyte> [#uses=1] > %matches9 = seteq ubyte %c8, 97 ; <bool> [#uses=1] > br bool %matches9, label %ret_true, label %regex2 > > regex2: ; preds = %regex6 > %c = load ubyte* %iter ; <ubyte> [#uses=1] > %matches = seteq ubyte %c, 98 ; <bool> [#uses=1] > br bool %matches, label %ret_true, label %loop_step > > loop_step: ; preds = %regex2 > %next = getelementptr ubyte* %iter, int 1 ; <ubyte*> > [#uses=1] > br label %loop_test > > loop_test: ; preds = %loop_step, %entry > %iter = phi ubyte* [ %begin, %entry ], [ %next, %loop_step ] > ; <ubyte*> [#uses=4] > %done = setgt ubyte* %iter, %scan_end ; <bool> [#uses=1] > br bool %done, label %ret_false, label %regex6 > > ret_true: ; preds = %regex2, %regex6 > ret bool true > > ret_false: ; preds = %loop_test > ret bool false > } > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >-Chris -- http://nondot.org/sabre/ http://llvm.org/
On Mar 16, 2006, at 8:47 PM, Chris Lattner wrote:> On Thu, 16 Mar 2006, Eric Kidd wrote: >> The duplicate loads appear at the top of the %regex6 and %regex2 >> blocks below. I've tried various alias analysis implementations >> either alone or in combination. > > LICM doesn't remove common subexpressions, also -load-vn doesn't > affect LICM. Try "-licm -load-vn -gcse" instead of "-load-vn -licm"That works! Thank you. The bytecodes look really good--not only are the loads eliminated, but the tests are actually reduced to a switch statement using '-anders-aa -load-vn -gcse -simplifycfg -instcombine': regex6: ; preds = %loop_test %c8 = load ubyte* %iter ; <ubyte> [#uses=1] switch ubyte %c8, label %loop_step [ ubyte 97, label %ret_true ubyte 98, label %ret_true ] Unfortunately, this generates really weird code on the LLVM 1.6 PowerPC backend: LBB_matches_1: ; regex6 lbz r4, 0(r3) LBB_matches_2: ; NodeBlock rlwinm r5, r4, 0, 24, 31 cmplwi cr0, r5, 98 blt cr0, LBB_matches_4 ; LeafBlock LBB_matches_3: ; LeafBlock1 rlwinm r4, r4, 0, 24, 31 cmpwi cr0, r4, 98 beq cr0, LBB_matches_8 ; ret_true b LBB_matches_5 ; NewDefault LBB_matches_4: ; LeafBlock rlwinm r4, r4, 0, 24, 31 cmpwi cr0, r4, 97 beq cr0, LBB_matches_8 ; ret_true LBB_matches_5: ; NewDefault LBB_matches_6: ; loop_step I'm particularly confused by the rlwinm instructions that keep turning up in PowerPC output, and the double test against 98. I don't have a problem or anything; I'm just trying to figure out what's going on. :-) In any case, many thanks for your help! Cheers, Eric