[LLVMbugs] [Bug 2313] New: ARM: Rd and Rm registers should be different in MUL on ARMv5

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=2313

           Summary: ARM: Rd and Rm registers should be different in MUL on
                    ARMv5
           Product: libraries
           Version: trunk
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Backend: ARM
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: jhw at conjury.org
                CC: llvmbugs at cs.uiuc.edu


The ARMv6 CPU allows the Rd and Rm registers to be the same in the MUL
instruction, but this isn't allowed on, for example the ARMv5 CPU.  As a
result, GAS will issue complaints about LLVM-generated ARM code.  Here's a
sample:

sample.s: Assembler messages:
sample.s:59: Rd and Rm should be different in mul
sample.s:63: Rd and Rm should be different in mul
sample.s:104: Rd and Rm should be different in mul
sample.s:108: Rd and Rm should be different in mul

These messages came from assembling the following LLVM-generated code:

        .globl  rand_r
        .align  2
rand_r:
        ldr r3, [r0]
        ldr r2, .LCPI3_0
        cmp r3, #0
        moveq r3, r2
        ldr r2, .LCPI3_1
        umull r1, r2, r3, r2
        sub r1, r3, r2
        add r2, r2, r1, lsr #1
        mov r2, r2, lsr #16
        mov r1, #197, 30 @ 788
        orr r1, r1, #2, 22 @ 2048
        mul r1, r2, r1
        ldr r12, .LCPI3_2
        mul r2, r2, r12          ; <------------ Here is line 59.
        sub r3, r3, r2
        mov r2, #167
        orr r2, r2, #65, 24 @ 16640
        mul r3, r3, r2            ; <------------ Here is line 63.
        sub r3, r3, r1
        sub r2, r3, #6, 2 @ -2147483647
        cmp r3, #0
        movge r2, r3
        str r2, [r0]
        bic r0, r2, #2, 2 @ -2147483648
        bx lr
.LBB3_1:
        .LCPI3_0:
        .long   123459876

        .LCPI3_1:
        .long   110892733

        .LCPI3_2:
        .long   127773

Yes, this really is the rand_r() function from the standard C library.

This code is fine for ARMv6, but it doesn't work for the ARMv5 CPU I'm playing
with, so here is the bug report.


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