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4db527032a
Summary:
In RewriteLoopExitValues, before expanding out an SCEV expression using
SCEVExpander, try to see if an existing LLVM IR expression already
computes the value we're interested in. If so use that existing
expression.
Apart from reducing IndVars' reliance on the rest of the compilation
pipeline, this also prevents IndVars from concluding some expressions as
"high cost" when they're not. For instance,
`InductiveRangeCheckElimination` often emits code of the following form:
```
len = umin(len_A, len_B)
loop:
...
if (i++ < len)
goto loop
outside_loop:
use(i)
```
`SCEVExpander` refuses to rewrite the use of `i` in `outside_loop`,
since it thinks the value of `i` on loop exit, `len`, is a high cost
expansion since it contains an `umax` in it. With this change,
`IndVars` can see that it can re-use `len` instead of creating a new
expression to compute `umin(len_A, len_B)`.
I considered putting this cleverness in `SCEVExpander`, but I was
worried that it may then have a deterimental effect on other passes
that use it. So I decided it was better to just do this in the one
place where it seems like an obviously good idea, with the intent of
generalizing later if needed.
Reviewers: atrick, reames
Subscribers: llvm-commits
Differential Revision: http://reviews.llvm.org/D10782
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241838 91177308-0d34-0410-b5e6-96231b3b80d8
37 lines
1.2 KiB
LLVM
37 lines
1.2 KiB
LLVM
; RUN: opt -S -indvars < %s | FileCheck %s
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define void @f(i32 %length.i.88, i32 %length.i, i8* %tmp12, i32 %tmp10, i8* %tmp8) {
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; CHECK-LABEL: @f(
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not_zero11.preheader:
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%tmp13 = icmp ugt i32 %length.i, %length.i.88
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%tmp14 = select i1 %tmp13, i32 %length.i.88, i32 %length.i
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%tmp15 = icmp sgt i32 %tmp14, 0
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br i1 %tmp15, label %not_zero11, label %not_zero11.postloop
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not_zero11:
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%v_1 = phi i32 [ %tmp22, %not_zero11 ], [ 0, %not_zero11.preheader ]
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%tmp16 = zext i32 %v_1 to i64
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%tmp17 = getelementptr inbounds i8, i8* %tmp8, i64 %tmp16
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%tmp18 = load i8, i8* %tmp17, align 1
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%tmp19 = zext i8 %tmp18 to i32
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%tmp20 = or i32 %tmp19, %tmp10
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%tmp21 = trunc i32 %tmp20 to i8
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%addr22 = getelementptr inbounds i8, i8* %tmp12, i64 %tmp16
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store i8 %tmp21, i8* %addr22, align 1
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%tmp22 = add nuw nsw i32 %v_1, 1
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%tmp23 = icmp slt i32 %tmp22, %tmp14
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br i1 %tmp23, label %not_zero11, label %main.exit.selector
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main.exit.selector:
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; CHECK-LABEL: main.exit.selector:
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; CHECK: %tmp24 = icmp slt i32 %tmp14, %length.i
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%tmp24 = icmp slt i32 %tmp22, %length.i
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br i1 %tmp24, label %not_zero11.postloop, label %leave
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leave:
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ret void
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not_zero11.postloop:
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ret void
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}
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