[Bug rtl-optimization/23726] New: Missed optimizations for divmod

[bjoern dot m dot haase at web dot de]

Prologue: 
 
Purpose of this enhancement request is not only to resolve a missed 
optimization issue for the avr port. It aims to show up ways for resolving a 
general difficulty for the RTL optimizers that we are presently having for 
double-set insn: A difficulty that shows up when dealing with side effects of 
double-set RTL insns that produce unexpected useful results.  
 
IMO this is a general issue that will not only show up for the divmod4 patterns 
but will probably also be very important for the efficiency of code generated 
after the CCmode transition for targets where 1.) arithmetic or logic 
operations leave the CC register with useful information and 2.) arithmetic 
operations on SImode or DImode objects need to be lowered to a sequence of 
subreg operations. 
 
At the end of this bug report I'm about to ask for an early CSE pass or a small 
change of the jump2 pass. 
 
 
Text body: 
 
When having a function like 
 
uint32_t d,m; 
void foo (uint32_t z, uint32_t n) 
{ 
  d = z / n; 
  m = z % n; 
} 
 
the avr port presently does not recognize that it needs to call the udivmodsi 
function only once. Assembly code generated reads, thus,  
        movw r14,r22 
        movw r16,r24 
        movw r10,r18 
        movw r12,r20 
        call __udivmodsi4 
        sts d,r18 
        sts (d)+1,r19 
        sts (d)+2,r20 
        sts (d)+3,r21 
        movw r24,r16 
        movw r22,r14 
        movw r20,r12 
        movw r18,r10 
        call __udivmodsi4 
        sts m,r22 
        sts (m)+1,r23 
        sts (m)+2,r24 
        sts (m)+3,r25 
(function prologue/epilogue stripped). The issue is that the CSE passes 
presently are not able to realize that the same result is calculated twice. The 
optimizers do, thus, not recognize that the first divmod4 call had a useful 
side-effect (the one that it calculates as well the mod). 
This could be fixed by attaching appropriate REG_EQUAL register notes to the 
insns that copy the results from the library call that reside in hard regs to 
target pseudos. The present implementation of the avr port does not generate 
such notes since it generates the calls explicitly and not by optabs.c (because 
the implementation knows exactly which registers will be clobbered by the 
library call). I will attach a patch that adds these required REG_EQUAL notes.  
 
The important issue is, that the REG_EQUAL note needs to refer to the original 
pseudos that were used as input parameters for the library call. Otherwise CSE 
will not realize the optimization opportunity. 
 
When using the attached patch, i.e. when adding the appropriate REG_EQUAL notes 
to the RTL, I veryfied that CSE is able to realize the optimization 
opportunity. I.e. one ends up with assembly output reading 
        call __udivmodsi4 
        sts d,r18 
        sts (d)+1,r19 
        sts (d)+2,r20 
        sts (d)+3,r21 
        sts m,r22 
        sts (m)+1,r23 
        sts (m)+2,r24 
        sts (m)+3,r25 
. The only difficulty is, that with the present optimization setup, the jump2 
pass removes the insn that contain the useful REG_EQUAL notes for the 
unexpected useful side-effect because it correctly recognizes that it is 
"trivially dead". For this reason, I had to place the first CSE pass before the 
jump2 pass in "passes.c" in order to get above results. I.e. my "passes.c" 
excerpt reads 
  NEXT_PASS (pass_instantiate_virtual_regs); 
  NEXT_PASS (pass_cse); 
  NEXT_PASS (pass_jump2); 
instead of  
  NEXT_PASS (pass_instantiate_virtual_regs); 
  NEXT_PASS (pass_jump2); 
  NEXT_PASS (pass_cse); 
. 
 
Epilogue: 
 
For this reason the attached patch could only resolve this issue IMO only if 
one of the following solutions is realized: 
 
1.) the jump2 pass is changed such that it refrains from deleting trivially 
dead insn if they are containing REG_EQUAL notes because possibly it could be 
necessary to make them being seen at least once by a CSE pass. 
2.) an earlier CSE pass is added to the optimizer flow. Possibly controlled by 
using a target-dependent flag. 
 
Yours, 
 
Bjoern

-- 
           Summary: Missed optimizations for divmod
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P1
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bjoern dot m dot haase at web dot de
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: unknown-x86_64-linux
  GCC host triplet: unknown-x86_64-linux
GCC target triplet: avr-*-*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23726