Changeset 278769 in webkit

Timestamp:

Jun 11, 2021, 11:26:55 AM (5 years ago)

Author:

commit-queue@webkit.org

Message:

Air ARM64 sub32 opcode should indicate that it zero-extends its result
​https://bugs.webkit.org/show_bug.cgi?id=226849

Patch by Yijia Huang <Yijia Huang> on 2021-06-11
Reviewed by Saam Barati.

Sub32 was previously not saying that its result is zero defined. However,
sub32 on arm64 architectures zero defines its result, so the top 32 bits
are zeroed. The issue with this is what we were not eliding provably
redundant zero extend operations.

For example:
B3 IR
@0 = Trunc(ArgumentReg(0))
@1 = Trunc(ArgumentReg(1))
@2 = Sub(@0, @1)
@3 = ZExt32(@2) i64
@4 = Return(@3) or Store(@3, @x)

Old optimized Air IR
Sub32 %x0, %x1, %x0, b@2
Move32 %x0, %x0, b@3
Ret64 %x0, b@4 or Move %x0, (%x), b@4

To remove that redundant zero extend instruction (Move32), Air arm64
sub32 opcode should indicate that it zero-extends its result.

New optimized Air IR
Sub32 %x0, %x1, %x0, b@2
Ret64 %x0, b@6 or Move %x0, (%x), b@4

• b3/air/AirOpcode.opcodes:

Location:

trunk/Source/JavaScriptCore

Files:

• ChangeLog (modified) (1 diff)
• b3/air/AirOpcode.opcodes (modified) (1 diff)

trunk/Source/JavaScriptCore/ChangeLog

@@ +1 @@
+2021-06-11  Yijia Huang  <yijia_huang@apple.com>
+
+        Air ARM64 sub32 opcode should indicate that it zero-extends its result
+        https://bugs.webkit.org/show_bug.cgi?id=226849
+
+        Reviewed by Saam Barati.
+
+        Sub32 was previously not saying that its result is zero defined. However,
+        sub32 on arm64 architectures zero defines its result, so the top 32 bits
+        are zeroed. The issue with this is what we were not eliding provably
+        redundant zero extend operations.
+
+        For example:
+        // B3 IR
+        @0 = Trunc(ArgumentReg(0))
+        @1 = Trunc(ArgumentReg(1))
+        @2 = Sub(@0, @1)
+        @3 = ZExt32(@2) // i64
+        @4 = Return(@3) or Store(@3, @x)
+
+        // Old optimized Air IR
+        Sub32 %x0, %x1, %x0, b@2
+        Move32 %x0, %x0, b@3
+        Ret64 %x0, b@4 or Move %x0, (%x), b@4
+
+        To remove that redundant zero extend instruction (Move32), Air arm64 
+        sub32 opcode should indicate that it zero-extends its result.
+
+        // New optimized Air IR
+        Sub32 %x0, %x1, %x0, b@2
+        Ret64 %x0, b@6 or Move %x0, (%x), b@4
+
+        * b3/air/AirOpcode.opcodes:
+
 2021-06-10  Keith Miller  <keith_miller@apple.com>
 

trunk/Source/JavaScriptCore/b3/air/AirOpcode.opcodes

@@ -178 +178 @@
     x86: Tmp, Index
 
-arm64: Sub32 U:G:32, U:G:32, D:G:32
+arm64: Sub32 U:G:32, U:G:32, ZD:G:32
     Tmp, Tmp, Tmp