llvm dev - Apr 2019 - Loop Strength Reduction Pass Does Not Work for Some Varialbles Related to Induction Variables

Dear Momchil,

    Thanks a lot for your prompt reply and kindly suggestion.
    The code will truly lead to some automatic unrolling with GCC and increase
the number of operations in the assembly code. However, I am focus on the the IR
optimization and the calculation of the array element offest, which may need
multiplication if the IR instruction GEP is lowered to arithmetic operations.
The multiplication operations should be able to be removed by using the LLVM
Pass LoopStrengthReduction. Interestingly, it does not work for the test code.
    I wamt to post the IR code but it might be too long to be in the mail.
    Thanks again for your time and precious suggestions!

Best regards,
------------------------------------------
Tingyuan LIANG
MPhil Student
Department of Electronic and Computer Engineering
The Hong Kong University of Science and Technology



发件人: Momchil Velikov
发送时间: 4月16日星期二 01:13
主题: Re: [llvm-dev] Loop Strength Reduction Pass Does Not Work for Some
Varialbles Related to Induction Variables
收件人: Tingyuan LIANG
抄送: llvm-dev at lists.llvm.org


From a quick glance at the code, it looks like both inner loops were unrolled at
least once (vary by architecture)
and have the expected number of multiplications, four and two (if unrolled
once), respectively.

https://gcc.godbolt.org/z/1ks4Pt

--
Momchil Velikov,
Senior Compiler Engineer
Arm



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Some multiplications, used in address calculation, may stay after loop
strength reduction, if the target machine has scaled index addressing mode.

--
Momchil Velikov,
Senior Compiler Engineer
Arm
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Dear Momchil,

    Thanks a lot for your detailed information and yes, it seems that loop
strength reduction is limited by scaled index addressing mode. Now, I implement
a simple LSR pass to do LSR aggressively, ignoring the scaled addressing mode.
    Thanks again for you time and suggestions!

Best regards,
------------------------------------------
Tingyuan LIANG
MPhil Student
Department of Electronic and Computer Engineering
The Hong Kong University of Science and Technology
________________________________
From: Momchil Velikov <momchil.velikov at gmail.com>
Sent: Tuesday, April 16, 2019 2:40 AM
To: Tingyuan LIANG
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Loop Strength Reduction Pass Does Not Work for Some
Varialbles Related to Induction Variables


Some multiplications, used in address calculation, may stay after loop strength
reduction, if the target machine has scaled index addressing mode.

--
Momchil Velikov,
Senior Compiler Engineer
Arm

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