[PATCH] [ARM] Add Thumb-2 code size optimization (wide CMP to narrow ADD)m

[Tilmann Scheller]

Hi,

attached is a patch for a small Thumb-2 code size optimization I 
implemented a while ago.

Tested with r235577.

Feedback welcome :)

Regards,

Tilmann



[ARM] Add Thumb-2 code size optimization (wide CMP to narrow ADD)

For wide CMP instructions which are comparing against a small negative 
immediate in the range of -255 to -1 it is possible to replace them with 
narrow ADD instructions instead.

Apart from the constraints on the immediate value there are additional 
conditions which need to be met for this transformation to be safe:
   - The CMP register operand needs to be in a low register and there 
can't be subsequent uses of the register as ADD will clobber it
   - Any uses of the CPSR def by the CMP instruction are restricted to 
the Z flag only

This code size optimization opportunity was spotted in an inner loop of 
175.vpr.

Example:

     define i32 @foo(i32 %a) nounwind {
     entry:
       %cmp = icmp eq i32 %a, -1
       br i1 %cmp, label %if.then, label %if.else

     if.then:
       ret i32 42

     if.else:
       ret i32 13
     }

     This translates into the following code sequence:

     movs    r1, #13                 @ encoding: [0x0d,0x21]
     cmp.w   r0, #-1                 @ encoding: [0xb0,0xf1,0xff,0x3f]
     it      eq                      @ encoding: [0x08,0xbf]
     moveq   r1, #42                 @ encoding: [0x2a,0x21]
     mov     r0, r1                  @ encoding: [0x08,0x46]
     bx      lr                      @ encoding: [0x70,0x47]

     With the code size optimization applied we get the following code 
sequence:

     movs    r1, #13                 @ encoding: [0x0d,0x21]
     adds    r0, #1                  @ encoding: [0x01,0x30]
     it      eq                      @ encoding: [0x08,0xbf]
     moveq   r1, #42                 @ encoding: [0x2a,0x21]
     mov     r0, r1                  @ encoding: [0x08,0x46]
     bx      lr                      @ encoding: [0x70,0x47]
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 0001-ARM-Add-Thumb-2-code-size-optimization-wide-CMP-to-n.patch
Type: text/x-patch
Size: 6909 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150423/bb0d7ed9/attachment.bin>