[AArch64][GlobalISel] Add heuristics for localizing G_CONSTANT.

This adds similar heuristics to G_GLOBAL_VALUE, querying the cost of
materializing a specific constant in code size. Doing so prevents us from
sinking constants which require multiple instructions to generate into
use blocks.

Code size savings on CTMark -Os:
Program                                       size.__text
                                              before         after           diff
ClamAV/clamscan                               381940.00      382052.00       0.0%
lencod/lencod                                 428408.00      428428.00       0.0%
SPASS/SPASS                                   411868.00      411876.00       0.0%
kimwitu++/kc                                  449944.00      449944.00       0.0%
Bullet/bullet                                 463588.00      463556.00      -0.0%
sqlite3/sqlite3                               284696.00      284668.00      -0.0%
consumer-typeset/consumer-typeset             414492.00      414424.00      -0.0%
7zip/7zip-benchmark                           595244.00      594972.00      -0.0%
mafft/pairlocalalign                          247512.00      247368.00      -0.1%
tramp3d-v4/tramp3d-v4                         372884.00      372044.00      -0.2%
                           Geomean difference                               -0.0%

Differential Revision: https://reviews.llvm.org/D130554

1 files changed, 1 insertions, 14 deletions

diff --git a/llvm/lib/CodeGen/TargetLoweringBase.cpp b/llvm/lib/CodeGen/TargetLoweringBase.cpp
index a342a4d..517e835 100644
--- a/llvm/lib/CodeGen/TargetLoweringBase.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringBase.cpp
@@ -2335,18 +2335,6 @@ bool TargetLoweringBase::shouldLocalize(const MachineInstr &MI,
     llvm_unreachable("Unexpected remat cost");
   };
 
-  // Helper to walk through uses and terminate if we've reached a limit. Saves
-  // us spending time traversing uses if all we want to know is if it's >= min.
-  auto isUsesAtMost = [&](unsigned Reg, unsigned MaxUses) {
-    unsigned NumUses = 0;
-    auto UI = MRI.use_instr_nodbg_begin(Reg), UE = MRI.use_instr_nodbg_end();
-    for (; UI != UE && NumUses < MaxUses; ++UI) {
-      NumUses++;
-    }
-    // If we haven't reached the end yet then there are more than MaxUses users.
-    return UI == UE;
-  };
-
   switch (MI.getOpcode()) {
   default:
     return false;
@@ -2363,8 +2351,7 @@ bool TargetLoweringBase::shouldLocalize(const MachineInstr &MI,
     unsigned MaxUses = maxUses(RematCost);
     if (MaxUses == UINT_MAX)
       return true; // Remats are "free" so always localize.
-    bool B = isUsesAtMost(Reg, MaxUses);
-    return B;
+    return MRI.hasAtMostUserInstrs(Reg, MaxUses);
   }
   }
 }