commit 79e6eb8b79be6249358b7801bc511290dacf10d0
author Mathieu Chartier <mathieuc@google.com> Mon Aug 26 12:33:46 2019 -0700
committer Mathieu Chartier <mathieuc@google.com> Thu Aug 29 16:05:52 2019 +0000
tree 1a04d214dd6223423abd442d8d9b0b61a3db2336
parent bae88c0759d48acf29b58d960ad2665e3462dfda

Prevent overflow for AOT hotness counters

Previous, the addition did not have a check for overflow and might wrap
around since the counter is only 16 bits.

Modified the test to exercise this.

The slowdown from fixing the overflow is 2% average on golem arm32/64.
Overall this brings the slowdown from the counter to ~15% from ~13%.

The benchmarks that regress the most are loopy ones that I would
consider non-representative. Code size increases by 0.6%.

Bug: 139883463
Test: test/run-test --host --64 --prebuild 674-hotness-compiled
Test: test/run-test --host --prebuild 674-hotness-compiled
Test: test/run-test --64 --prebuild 674-hotness-compiled
Test: test/run-test ---prebuild 674-hotness-compiled

Change-Id: Icf0ab2aedbc40ab10c9d952ce0f9c7b5e5feaf15

compiler/optimizing/code_generator_arm64.cc

diff --git a/compiler/optimizing/code_generator_arm64.cc b/compiler/optimizing/code_generator_arm64.cc
index a299ece..269b4e9 100644
--- a/compiler/optimizing/code_generator_arm64.cc
+++ b/compiler/optimizing/code_generator_arm64.cc
@@ -1068,8 +1068,10 @@
   if (GetCompilerOptions().CountHotnessInCompiledCode()) {
     UseScratchRegisterScope temps(masm);
     Register temp = temps.AcquireX();
-    __ Ldrh(temp, MemOperand(kArtMethodRegister, ArtMethod::HotnessCountOffset().Int32Value()));
-    __ Add(temp, temp, 1);
+    __ Ldrsh(temp, MemOperand(kArtMethodRegister, ArtMethod::HotnessCountOffset().Int32Value()));
+    __ Adds(temp, temp, 1);
+    // Subtract carry if it overflowed.
+    __ Sbc(temp, temp, xzr);
     __ Strh(temp, MemOperand(kArtMethodRegister, ArtMethod::HotnessCountOffset().Int32Value()));
   }
 
@@ -3175,8 +3177,10 @@
       Register temp1 = temps.AcquireX();
       Register temp2 = temps.AcquireX();
       __ Ldr(temp1, MemOperand(sp, 0));
-      __ Ldrh(temp2, MemOperand(temp1, ArtMethod::HotnessCountOffset().Int32Value()));
-      __ Add(temp2, temp2, 1);
+      __ Ldrsh(temp2, MemOperand(temp1, ArtMethod::HotnessCountOffset().Int32Value()));
+      __ Adds(temp2, temp2, 1);
+      // Subtract carry if it overflowed.
+      __ Sbc(temp2, temp2, xzr);
       __ Strh(temp2, MemOperand(temp1, ArtMethod::HotnessCountOffset().Int32Value()));
     }
     GenerateSuspendCheck(info->GetSuspendCheck(), successor);