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Re: Unexpected instruction sequence on MIPS

From: Ryan Mansfield <rmansfield at qnx dot com>
To: "Aaron W. LaFramboise" <aaronavay62 at aaronwl dot com>
Cc: gcc-help at gcc dot gnu dot org
Date: Tue, 14 Nov 2006 07:23:57 -0500
Subject: Re: Unexpected instruction sequence on MIPS
References: <45599432.60502@aaronwl.com>

Aaron W. LaFramboise wrote:

On a GCC 4.1.1 cross targeted to mips-mips-elf, this C input file:
int sum(int a, int b) {
  return a + b + 42;
}
produces this output with -O3:
addiu   v0,a0,42
jr      ra
addu    v0,a1,v0
Why does the compiler place "jr" before "addu"?

The compiler reorders the instructions to take advantage of the branch delay slot. Take a look at the output with -fno-delayed-branch.

Regards,

Ryan Mansfield

References:
- Unexpected instruction sequence on MIPS
  - From: Aaron W. LaFramboise

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