Add comments to bug fix code.
R=sgjesse@google.com
Change-Id: I5f19d19ca6c38ba156363a9e513282e6815fe6e7
diff --git a/src/main/java/com/android/tools/r8/ir/optimize/CodeRewriter.java b/src/main/java/com/android/tools/r8/ir/optimize/CodeRewriter.java
index 03e314c..3feb59a 100644
--- a/src/main/java/com/android/tools/r8/ir/optimize/CodeRewriter.java
+++ b/src/main/java/com/android/tools/r8/ir/optimize/CodeRewriter.java
@@ -1846,6 +1846,9 @@
@Override
protected boolean doEquivalent(Instruction a, Instruction b) {
+ // Some Dalvik VMs incorrectly handle Cmp instructions which leads to a requirement
+ // that we do not perform common subexpression elimination for them. See comment on
+ // canHaveCmpLongBug for details.
if (a.isCmp() && code.options.canHaveCmpLongBug()) {
return false;
}
diff --git a/src/main/java/com/android/tools/r8/utils/InternalOptions.java b/src/main/java/com/android/tools/r8/utils/InternalOptions.java
index cc8c9ce..9b893d1 100644
--- a/src/main/java/com/android/tools/r8/utils/InternalOptions.java
+++ b/src/main/java/com/android/tools/r8/utils/InternalOptions.java
@@ -515,6 +515,32 @@
// Some dalvik versions found in the wild perform invalid JIT compilation of cmp-long
// instructions where the result register overlaps with the input registers.
// See b/74084493.
+ //
+ // The same dalvik versions also have a bug where the JIT compilation of code such as:
+ //
+ // void method(long l) {
+ // if (l < 0) throw new RuntimeException("less than");
+ // if (l == 0) throw new RuntimeException("equal");
+ // }
+ //
+ // Will enter the case for l==0 even when l is non-zero. The code generated for this is of
+ // the form:
+ //
+ // 0: 0x00: ConstWide16 v0, 0x0000000000000000 (0)
+ // 1: 0x02: CmpLong v2, v4, v0
+ // 2: 0x04: IfLtz v2, 0x0c (+8)
+ // 3: 0x06: IfNez v2, 0x0a (+4)
+ //
+ // However, the jit apparently clobbers the input register in the IfLtz instruction. Therefore,
+ // for dalvik VMs we have to instead generate the following code:
+ //
+ // 0: 0x00: ConstWide16 v0, 0x0000000000000000 (0)
+ // 1: 0x02: CmpLong v2, v4, v0
+ // 2: 0x04: IfLtz v2, 0x0e (+10)
+ // 3: 0x06: CmpLong v2, v4, v0
+ // 4: 0x08: IfNez v2, 0x0c (+4)
+ //
+ // See b/75408029.
public boolean canHaveCmpLongBug() {
return minApiLevel < AndroidApiLevel.L.getLevel();
}
