optimization/3977: arm peephole for loading two consecutive memory locations generates suboptimal code (on arm7tdmi)

[segher@chello.nl]

>Number:         3977
>Category:       optimization
>Synopsis:       arm peephole for loading two consecutive memory locations generates suboptimal code (on arm7tdmi)
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Thu Aug 09 12:46:01 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     segher@chello.nl
>Release:        3.0
>Organization:
>Environment:

>Description:
I often see generated code like

add   temp, pointer, #offset
ldmia temp, {regA, regB}

(after which temp is dead)

which is slower than just

ldr regA, [pointer, #offset]
ldr regB, [pointer, #offset+4]

and wastes a register as well
>How-To-Repeat:
a lot of code will do this
>Fix:
change the peephole, i think
>Release-Note:
>Audit-Trail:
>Unformatted: