[LLVMbugs] [Bug 22598] New: Passing big array to some function makes compilation very slow

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Sun Feb 15 09:59:52 PST 2015


            Bug ID: 22598
           Summary: Passing big array to some function makes compilation
                    very slow
           Product: libraries
           Version: 3.5
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: lin90162 at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


When I was developing my compiler with LLVM,  I noticed the case when
compilation didn't finish.  I reduced the LLVM IR which my compiler emitted as

;;  Start  ;;

; ModuleID = 'foo.dcs'
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-darwin13.4.0"

; Function Attrs: nounwind
define i32 @foo([30000 x i64] %x) #0 {
  ret i32 0

; Function Attrs: nounwind
define i32 @main() #0 {
  %0 = alloca [30000 x i64]
  %1 = bitcast [30000 x i64]* %0 to i8*
  call void @llvm.memset.p0i8.i64(i8* %1, i8 0, i64 240000, i32 8, i1 false)
  %2 = load [30000 x i64]* %0
  %3 = call i32 @foo([30000 x i64] %2)
  ret i32 0

; Function Attrs: nounwind
declare void @llvm.memset.p0i8.i64(i8* nocapture, i8, i64, i32, i1) #0

attributes #0 = { nounwind }

;;  end  ;;

When above code was compiled with llc like below,

$ llc foo.ll

llc took too much time and I ended up with terminating the process.  It seemed
that passing [30000 x i64] made compilation very slow because the issue didn't
occur when I removed calling @foo() and definition of @foo().  And it seemed
not to go into infinite loop because compilation finished when I reduced the
size of array (e.g. 30000 -> 3000).

As I described above, this issue occurred on my compiler at first.  So I tried
debugging with lldb in my desktop.  As the result, it occured in
hasNUsesOfValue() member function of SDNode class which is called by run()
member function of llvm::PassManager.  lldb also said that it entered for loop
in hasNUsesOfValue() very often.


At last my environment is below:

- LLVM (http://llvm.org/):
    LLVM version 3.5.0
    Optimized build with assertions.
    Built Sep  6 2014 (01:11:29).
    Default target: x86_64-apple-darwin13.4.0
    Host CPU: core-avx2
- iMac 2013 mid

And I confirmed that LLC 3.5.1 in Linux occurred the same problem with above
LLVM IR code.

As reference, my compiler is below:


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