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https://github.com/c64scene-ar/llvm-6502.git
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3071363bcd
discussions with Andy. Fundamentally, the previous algorithm is both counter productive on several fronts and prioritizing things which aren't necessarily the most important: static branch prediction. The new algorithm uses the existing loop CFG structure information to walk through the CFG itself to layout blocks. It coalesces adjacent blocks within the loop where the CFG allows based on the most likely path taken. Finally, it topologically orders the block chains that have been formed. This allows it to choose a (mostly) topologically valid ordering which still priorizes fallthrough within the structural constraints. As a final twist in the algorithm, it does violate the CFG when it discovers a "hot" edge, that is an edge that is more than 4x hotter than the competing edges in the CFG. These are forcibly merged into a fallthrough chain. Future transformations that need te be added are rotation of loop exit conditions to be fallthrough, and better isolation of cold block chains. I'm also planning on adding statistics to model how well the algorithm does at laying out blocks based on the probabilities it receives. The old tests mostly still pass, and I have some new tests to add, but the nested loops are still behaving very strangely. This almost seems like working-as-intended as it rotated the exit branch to be fallthrough, but I'm not convinced this is actually the best layout. It is well supported by the probabilities for loops we currently get, but those are pretty broken for nested loops, so this may change later. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142743 91177308-0d34-0410-b5e6-96231b3b80d8
142 lines
3.4 KiB
LLVM
142 lines
3.4 KiB
LLVM
; RUN: llc -march=x86 -enable-block-placement < %s | FileCheck %s
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declare void @error(i32 %i, i32 %a, i32 %b)
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define i32 @test_ifchains(i32 %i, i32* %a, i32 %b) {
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; Test a chain of ifs, where the block guarded by the if is error handling code
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; that is not expected to run.
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; CHECK: test_ifchains:
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; CHECK: %entry
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; CHECK: %else1
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; CHECK: %else2
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; CHECK: %else3
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; CHECK: %else4
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; CHECK: %exit
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; CHECK: %then1
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; CHECK: %then2
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; CHECK: %then3
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; CHECK: %then4
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; CHECK: %then5
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entry:
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%gep1 = getelementptr i32* %a, i32 1
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%val1 = load i32* %gep1
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%cond1 = icmp ugt i32 %val1, 1
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br i1 %cond1, label %then1, label %else1, !prof !0
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then1:
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call void @error(i32 %i, i32 1, i32 %b)
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br label %else1
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else1:
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%gep2 = getelementptr i32* %a, i32 2
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%val2 = load i32* %gep2
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%cond2 = icmp ugt i32 %val2, 2
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br i1 %cond2, label %then2, label %else2, !prof !0
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then2:
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call void @error(i32 %i, i32 1, i32 %b)
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br label %else2
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else2:
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%gep3 = getelementptr i32* %a, i32 3
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%val3 = load i32* %gep3
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%cond3 = icmp ugt i32 %val3, 3
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br i1 %cond3, label %then3, label %else3, !prof !0
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then3:
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call void @error(i32 %i, i32 1, i32 %b)
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br label %else3
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else3:
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%gep4 = getelementptr i32* %a, i32 4
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%val4 = load i32* %gep4
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%cond4 = icmp ugt i32 %val4, 4
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br i1 %cond4, label %then4, label %else4, !prof !0
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then4:
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call void @error(i32 %i, i32 1, i32 %b)
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br label %else4
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else4:
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%gep5 = getelementptr i32* %a, i32 3
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%val5 = load i32* %gep5
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%cond5 = icmp ugt i32 %val5, 3
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br i1 %cond5, label %then5, label %exit, !prof !0
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then5:
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call void @error(i32 %i, i32 1, i32 %b)
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br label %exit
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exit:
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ret i32 %b
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}
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!0 = metadata !{metadata !"branch_weights", i32 4, i32 64}
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define i32 @test_loop_align(i32 %i, i32* %a) {
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; Check that we provide basic loop body alignment with the block placement
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; pass.
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; CHECK: test_loop_align:
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; CHECK: %entry
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; CHECK: .align [[ALIGN:[0-9]+]],
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; CHECK-NEXT: %body
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; CHECK: %exit
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entry:
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br label %body
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body:
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%iv = phi i32 [ 0, %entry ], [ %next, %body ]
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%base = phi i32 [ 0, %entry ], [ %sum, %body ]
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%arrayidx = getelementptr inbounds i32* %a, i32 %iv
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%0 = load i32* %arrayidx
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%sum = add nsw i32 %0, %base
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%next = add i32 %iv, 1
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%exitcond = icmp eq i32 %next, %i
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br i1 %exitcond, label %exit, label %body
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exit:
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ret i32 %sum
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}
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define i32 @test_nested_loop_align(i32 %i, i32* %a, i32* %b) {
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; Check that we provide nested loop body alignment.
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; CHECK: test_nested_loop_align:
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; CHECK: %entry
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; CHECK: .align [[ALIGN]],
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; CHECK-NEXT: %loop.body.2
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; CHECK: .align [[ALIGN]],
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; CHECK-NEXT: %inner.loop.body
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; CHECK-NOT: .align
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; CHECK: %exit
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entry:
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br label %loop.body.1
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loop.body.1:
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%iv = phi i32 [ 0, %entry ], [ %next, %loop.body.2 ]
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%arrayidx = getelementptr inbounds i32* %a, i32 %iv
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%bidx = load i32* %arrayidx
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br label %inner.loop.body
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inner.loop.body:
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%inner.iv = phi i32 [ 0, %loop.body.1 ], [ %inner.next, %inner.loop.body ]
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%base = phi i32 [ 0, %loop.body.1 ], [ %sum, %inner.loop.body ]
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%scaled_idx = mul i32 %bidx, %iv
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%inner.arrayidx = getelementptr inbounds i32* %b, i32 %scaled_idx
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%0 = load i32* %inner.arrayidx
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%sum = add nsw i32 %0, %base
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%inner.next = add i32 %iv, 1
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%inner.exitcond = icmp eq i32 %inner.next, %i
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br i1 %inner.exitcond, label %loop.body.2, label %inner.loop.body
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loop.body.2:
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%next = add i32 %iv, 1
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%exitcond = icmp eq i32 %next, %i
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br i1 %exitcond, label %exit, label %loop.body.1
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exit:
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ret i32 %sum
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}
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