Syoyo Fujita
2009-Jan-08 17:22 UTC
[LLVMdev] Loop elimination with floating point counter.
Hi Devang, Thanks. Yes, in the case variable i incremented by 1.0f is optimized. I don't know why... Anyway, I've filed this problem into bugzilla(Bug 3299) -- Syoyo On Fri, Jan 9, 2009 at 12:42 AM, Devang Patel <dpatel at apple.com> wrote:> > On Jan 8, 2009, at 4:36 AM, Syoyo Fujita wrote: > >> Hi LLVM-ers, >> >> I'd like to eliminate dead loop with floating point counter using >> LLVM, but the following loop wasn't optimized by opt. >> >> void >> func() { >> float i; >> for (i = 0.0f; i < 1000.0f; i += 1.2f) { >> } >> } > > FWIW, LLVM optimizer can eliminate this loop if i is incremented by 1.0f > > Pl. file a bugzilla report. > Thanks, > - > Devang >> >> >> $ clang -emit-llvm-bc floop.c >> $ opt -std-compile-opts floop.bc | llvm-dis >> >> define void @func(...) nounwind { >> entry: >> br label %forinc >> >> forinc: ; preds = %forinc, %entry >> %i.0.reg2mem.0 = phi float [ 0.000000e+00, %entry ], [ %add, %forinc >> ] ; <float> [#uses=1] >> %add = add float %i.0.reg2mem.0, 0x3FF3333340000000 ; <float> >> [#uses=2] >> %cmp = fcmp olt float %add, 1.000000e+03 ; <i1> [#uses=1] >> br i1 %cmp, label %forinc, label %afterfor >> >> afterfor: ; preds = %forinc >> ret void >> } >> >> What I expected is just one instruction "ret void" in function "func". >> >> Should I specify some specific optimizer pass for opt? >> If so, what is the right optimization pass to specify for opt to >> remove dead loop with floating point counter? >> >> I've tested some loop optimization pass, e.g. -licm, -loop-deletion, >> but nothing takes effect. >> >> FYI, gcc -O3 also can't optimize such a loop, but icc -O3 can. >> >> LLVM and clang used are recent version from svn. >> >> Thanks in advance. >> >> -- >> Syoyo >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >
Owen Anderson
2009-Jan-08 17:34 UTC
[LLVMdev] Loop elimination with floating point counter.
It's because with 1.0f, the loop index is simplified into an integer. With 1.2f, it isn't. The loop deletion pass is dependent on the loop analyses being able to determine that the loop is finite, which they don't attempt to do for loops with floating point indices. Attempting to do so would require additional reasoning about floating point precision issues. --Owen On Jan 8, 2009, at 9:22 AM, Syoyo Fujita wrote:> Hi Devang, > > Thanks. Yes, in the case variable i incremented by 1.0f is optimized. > I don't know why... > Anyway, I've filed this problem into bugzilla(Bug 3299) > > -- > Syoyo > > On Fri, Jan 9, 2009 at 12:42 AM, Devang Patel <dpatel at apple.com> > wrote: >> >> On Jan 8, 2009, at 4:36 AM, Syoyo Fujita wrote: >> >>> Hi LLVM-ers, >>> >>> I'd like to eliminate dead loop with floating point counter using >>> LLVM, but the following loop wasn't optimized by opt. >>> >>> void >>> func() { >>> float i; >>> for (i = 0.0f; i < 1000.0f; i += 1.2f) { >>> } >>> } >> >> FWIW, LLVM optimizer can eliminate this loop if i is incremented by >> 1.0f >> >> Pl. file a bugzilla report. >> Thanks, >> - >> Devang >>> >>> >>> $ clang -emit-llvm-bc floop.c >>> $ opt -std-compile-opts floop.bc | llvm-dis >>> >>> define void @func(...) nounwind { >>> entry: >>> br label %forinc >>> >>> forinc: ; preds = %forinc, %entry >>> %i.0.reg2mem.0 = phi float [ 0.000000e+00, %entry ], [ %add, >>> %forinc >>> ] ; <float> [#uses=1] >>> %add = add float %i.0.reg2mem.0, >>> 0x3FF3333340000000 ; <float> >>> [#uses=2] >>> %cmp = fcmp olt float %add, 1.000000e+03 ; >>> <i1> [#uses=1] >>> br i1 %cmp, label %forinc, label %afterfor >>> >>> afterfor: ; preds = %forinc >>> ret void >>> } >>> >>> What I expected is just one instruction "ret void" in function >>> "func". >>> >>> Should I specify some specific optimizer pass for opt? >>> If so, what is the right optimization pass to specify for opt to >>> remove dead loop with floating point counter? >>> >>> I've tested some loop optimization pass, e.g. -licm, -loop-deletion, >>> but nothing takes effect. >>> >>> FYI, gcc -O3 also can't optimize such a loop, but icc -O3 can. >>> >>> LLVM and clang used are recent version from svn. >>> >>> Thanks in advance. >>> >>> -- >>> Syoyo >>> _______________________________________________ >>> LLVM Developers mailing list >>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >> >> >> >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >> > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
Martin Geisse
2009-Jan-08 17:49 UTC
[LLVMdev] Loop elimination with floating point counter.
Isn't "simplifying" the loop index to an integer also dependent on
precision issues? The following loop is infinite:
for (float i=0.0; i<...; i+=1.0) {}
where ... is a large "integral" float literal, e.g. 2^40. At some
point, adding 1.0 to the loop index would not cause any changes
because of precision issues. However, if the float type is replaced
by some sufficiently large integer type, the loop terminates.
The necessary reasoning may be simpler though.
Greetings,
Martin Geisse
On Jan 8, 2009, at 6:34 PM, Owen Anderson wrote:
> It's because with 1.0f, the loop index is simplified into an integer.
> With 1.2f, it isn't. The loop deletion pass is dependent on the loop
> analyses being able to determine that the loop is finite, which they
> don't attempt to do for loops with floating point indices. Attempting
> to do so would require additional reasoning about floating point
> precision issues.
>
> --Owen
>
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