[LICM] Don't require optimized uses

LICM currently requests optimized use MSSA form. This is wasteful,
because LICM doesn't actually care about most uses, only those of
invariant pointers in loops. Everything else doesn't need to be
optimized.

LICM already uses the clobber walker in most places. This patch
adjusts one place that was using getDefiningAccess() to use it as
well, so we no longer have a dependence on pre-optimized uses.

This change is not NFC in that the fallback on the defining access
when there are too many clobber calls may now fall back to an
unoptimized use. In practice, I've not seen any problems with this
though. If desired, we could also increase licm-mssa-optimization-cap
to a higher value (increasing this from 100 to 200 has no impact on
average compile-time -- but also doesn't appear to have any impact
on LICM quality either).

This makes for a 0.9% geomean compile-time improvement on CTMark.

Differential Revision: https://reviews.llvm.org/D147437

Author: nikic

llvm/lib/Transforms/Scalar/LICM.cpp

@@ -401,7 +401,6 @@ bool LoopInvariantCodeMotion::runOnLoop(Loop *L, AAResults *AA, LoopInfo *LI,
   bool Changed = false;
 
   assert(L->isLCSSAForm(*DT) && "Loop is not in LCSSA form.");
-  MSSA->ensureOptimizedUses();
 
   // If this loop has metadata indicating that LICM is not to be performed then
   // just exit.
@@ -1304,7 +1303,8 @@ bool llvm::canSinkOrHoistInst(Instruction &I, AAResults *AA, DominatorTree *DT,
       if (auto *Accesses = MSSA->getBlockAccesses(BB)) {
         for (const auto &MA : *Accesses)
           if (const auto *MU = dyn_cast<MemoryUse>(&MA)) {
-            auto *MD = MU->getDefiningAccess();
+            auto *MD = getClobberingMemoryAccess(*MSSA, BAA, Flags,
+                const_cast<MemoryUse *>(MU));
             if (!MSSA->isLiveOnEntryDef(MD) &&
                 CurLoop->contains(MD->getBlock()))
               return false;

llvm/test/Analysis/MemorySSA/pr43427.ll

@@ -19,7 +19,7 @@
 ; CHECK-NEXT: [[NO7]] = MemoryPhi({lbl2,[[NO8]]},{for.end,2})
 
 ; CHECK: cleanup:
-; CHECK-NEXT: MemoryUse([[NO7]])
+; CHECK-NEXT: MemoryUse([[NO2]])
 ; CHECK-NEXT:  %cleanup.dest = load i32, ptr undef, align 1
 
 ; CHECK: lbl1.backedge:

llvm/test/Analysis/MemorySSA/pr45927.ll

@@ -24,7 +24,7 @@
 ; CHECK-NEXT: store i16 %inc.i.lcssa, ptr @c, align 1
 ; CHECK-NEXT: ; [[NO2:.*]] = MemoryDef([[NO6]])
 ; CHECK-NEXT: store i16 1, ptr @a, align 1
-; CHECK-NEXT: ; MemoryUse([[NO2]])
+; CHECK-NEXT: ; MemoryUse([[NO6]])
 ; CHECK-NEXT: %tmp2 = load i16, ptr @c, align 1
 ; CHECK-NEXT: br label %g.exit
 

llvm/test/Analysis/MemorySSA/pr49859.ll

@@ -67,7 +67,7 @@ final.cleanup:                                          ; preds = %if.then, %for
   br label %for.end
 
 ; CHECK: for.end:
-; CHECK-NEXT: ; MemoryUse([[NO12]])
+; CHECK-NEXT: ; MemoryUse([[NO20]])
 ; CHECK-NEXT:  %3 = load i8, ptr %sum, align 1
 for.end:                                          ; preds = %final.cleanup
   %8 = load i8, ptr %sum, align 1