[LLVMbugs] [Bug 11215] New: Thumb 2: LLVM emits 32-bit instructions due to unnecessary distance between cmp and it

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Sun Oct 23 12:22:17 PDT 2011


           Summary: Thumb 2: LLVM emits 32-bit instructions due to
                    unnecessary distance between cmp and it
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: ARM
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: xocotl at gmail.com
                CC: llvmbugs at cs.uiuc.edu

Outside IT blocks, the MOV immediate instruction sets flags.
LLVM often avoids it when it could do something else instead, and this ends up
ballooning up code quite a bit - 0 and 1 being very common 8-bit constant.

In fact, this occurs for much more than just MOV. Here are some examples:
180:    f100 0001     add.w    r0, r0, #1    ; 0x1
186:    f04f 0009     mov.w    r0, #9    ; 0x9
1ea:    f145 0200     adc.w    r2, r5, #0    ; 0x0
1f4:    f04f 0600     mov.w    r6, #0    ; 0x0

Here is an example of why:
 1ee:    4283          cmp    r3, r0
 1f0:    f8cc 201c     str.w    r2, [ip, #28]
 1f4:    f04f 0600     mov.w    r6, #0    ; 0x0 <---
 1f8:    bf98          it    ls
 1fa:    2601          movls    r6, #1
 1fc:    428a          cmp    r2, r1
 1fe:    f04f 0500     mov.w    r5, #0    ; 0x0 <---
 202:    bfd8          it    le

cmp in both of these cases could be moved directly next to it, allowing mov.w
to be a 2-byte movs. Here's another example:

2c8:    2a00          cmp    r2, #0
 2ca:    bf01          itttt    eq
 2cc:    f8cc 8008     streq.w    r8, [ip, #8]
 2d0:    f8cc 100c     streq.w    r1, [ip, #12]
 2d4:    f04f 0e00     moveq.w    lr, #0    ; 0x0
 2d8:    460d          moveq    r5, r1
 2da:    f1be 0f00     cmp.w    lr, #0    ; 0x0
 2de:    f04f 0200     mov.w    r2, #0    ; 0x0 <---
 2e2:    bfb8          it    lt

Not only could cmp/itttt be turned into cbnz to save code space and processor
time, again we see a mov.w inserted between the cmp.w and it unnecessarily,
causing it to be 4 bytes instead of 2.

It would be nice if this could be fixed. The cmp are unnecessarily far from
their corresponding it.

I expect this affects performance as well as size, given that only so much code
gets retrieved at a time, and this increases it *a lot*.

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