[llvm-dev] Expected constant simplification not happening
Sanjay Patel via llvm-dev
llvm-dev at lists.llvm.org
Thu Feb 11 15:02:10 PST 2016
[cc'ing Zia]
We have this transform with -Os for some cases after:
http://reviews.llvm.org/rL244601
http://reviews.llvm.org/D11363
but something in this example is causing the transform to not trigger.
I filed a related bug here:
https://llvm.org/bugs/show_bug.cgi?id=24448
If you can file your test case(s) in a bug report, that would be the best
way to track progress on solving it. Thanks!
On Thu, Feb 11, 2016 at 9:58 AM, Nat! via llvm-dev <llvm-dev at lists.llvm.org>
wrote:
> Hi
>
> the appended IR code does not optimize to my liking :)
>
> this is the interesting part in x86_64, that got produced via clang -Os:
> ---
> movq -16(%r12), %rax
> movl -4(%rax), %ecx
> andl $2298949, %ecx ## imm = 0x231445
> cmpq $2298949, (%rax,%rcx) ## imm = 0x231445
> leaq 8(%rax,%rcx), %rax
> cmovneq %r15, %rax
> movl $2298949, %esi ## imm = 0x231445
> movq %r12, %rdi
> movq %r14, %rdx
> callq *(%rax)
> ---
>
>
> and clang -O3:
> ---
> movq -16(%r12), %rax
> movl -4(%rax), %ecx
> andl $2298949, %ecx ## imm = 0x231445
> cmpl $2298949, (%rax,%rcx) ## imm = 0x231445
> jne LBB1_4
> leaq 8(%rax,%rcx), %rax
> jmp LBB1_5
> .align 4, 0x90
> LBB1_4:
> movq %r15, %rax
> LBB1_5:
> movl $2298949, %esi ## imm = 0x231445
> movq %r12, %rdi
> movq %r14, %rdx
> callq *(%rax)
> ---
>
> As you can see in both cases the constant $2298949 is replicated 3 times.
> I would have expected something like the following code at least for -Os:
>
> ---
> movq -16(%r12), %rax
> movl $2298949, %esi ### **** move on up
> movl -4(%rax), %ecx
> andl %esi, %ecx ###
> cmpl %esi, (%rax,%rcx) ###
> leaq 8(%rax,%rcx), %rax
> cmovneq %r15, %rax
> movq %r12, %rdi
> movq %r14, %rdx
> callq *(%rax)
> ---
>
> It is much shorter (33 bytes vs. 42 bytes) and I would assume at least the
> same speed or better. This is with llvm 3.7.0. And yes this pains me at the
> moment :)
>
> Ciao
> Nat!
> ----
> ; ModuleID = 'optimize-fail.c'
> target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
> target triple = "x86_64-apple-macosx10.10.0"
>
> %struct._foo = type {}
> %struct._entry = type { i32, i32, i8* (%struct._foo*, i32, i8*)* }
> %struct._table = type { i64, i32, i32, [1 x %struct._entry] }
> %struct.test = type { %struct.__foo, i32 }
> %struct.__foo = type { %struct._dispatch }
> %struct._dispatch = type { %struct._entry*, i8* (%struct._foo*, i32, i8*)*
> }
>
> @str = private unnamed_addr constant [8 x i8] c"table_f\00"
> @str.2 = private unnamed_addr constant [11 x i8] c"dispatch_f\00"
>
> ; Function Attrs: nounwind
> declare void @llvm.lifetime.start(i64, i8* nocapture) #1
>
> ; Function Attrs: nounwind
> declare void @llvm.lifetime.end(i64, i8* nocapture) #1
>
> ; Function Attrs: noinline nounwind ssp uwtable
> define i8* @foo(%struct._foo* %obj, i32 %unused, i8* %value) #2 {
> entry:
> %tobool.i = icmp eq %struct._foo* %obj, null
> %0 = bitcast %struct._foo* %obj to i8*
> %arrayidx.i.i = getelementptr inbounds i8, i8* %0, i64 -16
> %entries2.i = bitcast i8* %arrayidx.i.i to %struct._entry**
> %f7.i = getelementptr inbounds i8, i8* %0, i64 -8
> br i1 %tobool.i, label %for.end, label %call.exit.preheader
>
> call.exit.preheader: ; preds = %entry
> br label %call.exit
>
> call.exit: ; preds =
> %call.exit.preheader, %call.exit
> %i.04 = phi i32 [ %inc, %call.exit ], [ 0, %call.exit.preheader ]
> %1 = load %struct._entry*, %struct._entry** %entries2.i, align 8, !tbaa
> !2
> %arrayidx1.i.i = getelementptr inbounds %struct._entry, %struct._entry*
> %1, i64 -1
> %2 = bitcast %struct._entry* %arrayidx1.i.i to %struct._table*
> %mask4.i = getelementptr inbounds %struct._table, %struct._table* %2,
> i64 0, i32 2
> %3 = load i32, i32* %mask4.i, align 4, !tbaa !7
> %and.i = and i32 %3, 2298949
> %idxprom.i = zext i32 %and.i to i64
> %4 = bitcast %struct._entry* %1 to i8*
> %arrayidx.i = getelementptr inbounds i8, i8* %4, i64 %idxprom.i
> %key5.i = bitcast i8* %arrayidx.i to i32*
> %5 = load i32, i32* %key5.i, align 4, !tbaa !11
> %cmp.i = icmp eq i32 %5, 2298949
> %f6.i = getelementptr inbounds i8, i8* %arrayidx.i, i64 8
> %cond.in.v.i = select i1 %cmp.i, i8* %f6.i, i8* %f7.i
> %cond.in.i = bitcast i8* %cond.in.v.i to i8* (%struct._foo*, i32, i8*)**
> %cond.i = load i8* (%struct._foo*, i32, i8*)*, i8* (%struct._foo*, i32,
> i8*)** %cond.in.i, align 8
> %call8.i = tail call i8* %cond.i(%struct._foo* %obj, i32 2298949, i8*
> %value) #1
> %inc = add nuw nsw i32 %i.04, 1
> %exitcond = icmp eq i32 %inc, 100
> br i1 %exitcond, label %for.end.loopexit, label %call.exit
>
> for.end.loopexit: ; preds = %call.exit
> %call8.i.lcssa = phi i8* [ %call8.i, %call.exit ]
> br label %for.end
>
> for.end: ; preds =
> %for.end.loopexit, %entry
> %rval.0.lcssa = phi i8* [ %0, %entry ], [ %call8.i.lcssa,
> %for.end.loopexit ]
> ret i8* %rval.0.lcssa
> }
>
>
> attributes #1 = { nounwind }
> attributes #2 = { noinline nounwind ssp uwtable
> "disable-tail-calls"="false" "less-precise-fpmad"="false"
> "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"
> "no-infs-fp-math"="false" "no-nans-fp-math"="false"
> "stack-protector-buffer-size"="8" "target-cpu"="core2"
> "target-features"="+cx16,+sse,+sse2,+sse3,+ssse3" "unsafe-fp-math"="false"
> "use-soft-float"="false" }
>
> !llvm.module.flags = !{!0}
> !llvm.ident = !{!1}
>
> ---
>
> _______________________________________________
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> llvm-dev at lists.llvm.org
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>
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