[llvm] r207828 - Fold strlen(expr ? "str1" : "str2") to x ? len1 : len2. This fires about 330 times in a bootstrap of clang.

[Nick Lewycky]

On 1 May 2014 23:53, Benjamin Kramer <benny.kra at gmail.com> wrote:

>
> On 02.05.2014, at 06:11, Nick Lewycky <nicholas at mxc.ca> wrote:
>
> > Author: nicholas
> > Date: Thu May  1 23:11:45 2014
> > New Revision: 207828
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=207828&view=rev
> > Log:
> > Fold strlen(expr ? "str1" : "str2") to x ? len1 : len2. This fires about
> 330 times in a bootstrap of clang.
>
> Is there anything preventing the strlen folding from being hoisted into
> the general constant folding logic where we also handle math functions? I
> think instcombine would give us this kind of folding for free then.
>

Yes, the select would need to be a constexpr, which it can't always be
because its condition variable may be an instruction.

Nick


> - Ben
> >
> > Modified:
> >    llvm/trunk/lib/Transforms/Utils/SimplifyLibCalls.cpp
> >    llvm/trunk/test/Transforms/InstCombine/strlen-1.ll
> >
> > Modified: llvm/trunk/lib/Transforms/Utils/SimplifyLibCalls.cpp
> > URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/SimplifyLibCalls.cpp?rev=207828&r1=207827&r2=207828&view=diff
> >
> ==============================================================================
> > --- llvm/trunk/lib/Transforms/Utils/SimplifyLibCalls.cpp (original)
> > +++ llvm/trunk/lib/Transforms/Utils/SimplifyLibCalls.cpp Thu May  1
> 23:11:45 2014
> > @@ -784,10 +784,25 @@ struct StrLenOpt : public LibCallOptimiz
> >     if (uint64_t Len = GetStringLength(Src))
> >       return ConstantInt::get(CI->getType(), Len-1);
> >
> > +    // strlen(x?"foo":"bars") --> x ? 3 : 4
> > +    if (SelectInst *SI = dyn_cast<SelectInst>(Src)) {
> > +      uint64_t LenTrue = GetStringLength(SI->getTrueValue());
> > +      uint64_t LenFalse = GetStringLength(SI->getFalseValue());
> > +      if (LenTrue && LenFalse) {
> > +        Context->emitOptimizationRemark(
> > +            "simplify-libcalls", *Caller, SI->getDebugLoc(),
> > +            "folded strlen(select) to select of constants");
> > +        return B.CreateSelect(SI->getCondition(),
> > +                              ConstantInt::get(CI->getType(),
> LenTrue-1),
> > +                              ConstantInt::get(CI->getType(),
> LenFalse-1));
> > +      }
> > +    }
> > +
> >     // strlen(x) != 0 --> *x != 0
> >     // strlen(x) == 0 --> *x == 0
> >     if (isOnlyUsedInZeroEqualityComparison(CI))
> >       return B.CreateZExt(B.CreateLoad(Src, "strlenfirst"),
> CI->getType());
> > +
> >     return nullptr;
> >   }
> > };
> >
> > Modified: llvm/trunk/test/Transforms/InstCombine/strlen-1.ll
> > URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/strlen-1.ll?rev=207828&r1=207827&r2=207828&view=diff
> >
> ==============================================================================
> > --- llvm/trunk/test/Transforms/InstCombine/strlen-1.ll (original)
> > +++ llvm/trunk/test/Transforms/InstCombine/strlen-1.ll Thu May  1
> 23:11:45 2014
> > @@ -5,6 +5,7 @@
> > target datalayout =
> "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128"
> >
> > @hello = constant [6 x i8] c"hello\00"
> > + at longer = constant [7 x i8] c"longer\00"
> > @null = constant [1 x i8] zeroinitializer
> > @null_hello = constant [7 x i8] c"\00hello\00"
> > @nullstring = constant i8 0
> > @@ -85,6 +86,17 @@ define i1 @test_simplify8() {
> > ; CHECK-NEXT: ret i1 false
> > }
> >
> > +define i32 @test_simplify9(i1 %x) {
> > +; CHECK-LABEL: @test_simplify9
> > +  %hello = getelementptr [6 x i8]* @hello, i32 0, i32 0
> > +  %longer = getelementptr [7 x i8]* @longer, i32 0, i32 0
> > +  %s = select i1 %x, i8* %hello, i8* %longer
> > +  %l = call i32 @strlen(i8* %s)
> > +; CHECK-NEXT: select i1 %x, i32 5, i32 6
> > +  ret i32 %l
> > +; CHECK-NEXT: ret
> > +}
> > +
> > ; Check cases that shouldn't be simplified.
> >
> > define i32 @test_no_simplify1() {
> >
> >
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