[LLVMbugs] [Bug 7696] New: SelectionDAGBuilder doing bad things on certain architectures

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Fri Jul 23 10:38:59 PDT 2010


           Summary: SelectionDAGBuilder doing bad things on certain
           Product: libraries
           Version: 2.7
          Platform: PC
        OS/Version: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: micah.villmow at amd.com
                CC: llvmbugs at cs.uiuc.edu

Created an attachment (id=5257)
 --> (http://llvm.org/bugs/attachment.cgi?id=5257)
Patch that adds a boolean where a backend can specify whether to disable
certain optimizations because jumps are expensive.

The selection dag builder has an ‘optimization’ added into the visitBr function
which makes assumptions that are not valid on all architectures. The problem is
The following function
kernel void cf_test(global int* a, int b, int c, int e)
        int d = 0;
        if (!b && c < e) {
                d = a + b;
        *a = d;
Is transformed into something equivalent to this:
Kernel void cf_test(global int* a, int b, int c, int e)
  Int d;
  If (b) {
    d = 0;
  } else {
    if (c < e) {
      d = a + b;
    } else {
      d = 0;
 *a = d;

by the visitBr code found in SelectionDAGBuilder::visitBr():1188.

However, if jumps are expensive or jumps are not supported and high level flow
control needs to be reconstructed. This is extremely inefficient. For example
on AMD GPU’s, a single flow control instruction can take 40 cycles to execute,
but an bit instruction, can be executed every cycle. So obviously the
assumptions made by this block of code are inefficient on AMD hardware. 
Increasing control flow has a direct impact on performance and removing the
extra ‘and’ or ‘or’ in order to short circuit the conditional evaluation does
not work for our target.

So in order to make this type of instruction rely more on target specific
information. I’ve added a new Boolean to the TargetLoweringInfo class called
JumpIsExpensive along with accessor functions.

Please review the patch and apply if acceptable.

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