Changeset 240241 in webkit for trunk/Source/JavaScriptCore/offlineasm/x86.rb

Timestamp:

Jan 21, 2019, 12:50:59 PM (6 years ago)

Author:

[email protected]

Message:

[JSC] sub op with 0 should be optimized
​https://bugs.webkit.org/show_bug.cgi?id=190751

Reviewed by Mark Lam.

LLInt sometimes emit subp 0, %rxx. For example, maxFrameExtentForSlowPathCall is 0 in X86_64, ARM64, and ARM64E.
So subp maxFrameExtentForSlowPathCall sp becomes subp 0, %rsp. While addp 0, %rsp is removed in offlineasm,
sub operation does not have such an optimization. This patch applies the same optimization to sub operation already
done in add operation. Since the CPU flags changed in offlineasm's these operations are not considered (if these flags
are required, we use special branch operations instead), this optimization is sane.

One problem is that zero-extension of the 32bit register in 64bit architecture. If the instruction emission is skipped,
this won't be happen. Currently, we align our sub to add operation: we skip emission in this case.

• offlineasm/arm64.rb:
• offlineasm/x86.rb:

File:

• trunk/Source/JavaScriptCore/offlineasm/x86.rb (modified) (2 diffs)

trunk/Source/JavaScriptCore/offlineasm/x86.rb

@@ -762 +762 @@
             raise unless operands[2].is_a? RegisterID
             if operands[0].value == 0
-                unless operands[1] == operands[2]
+                if operands[1] != operands[2]
                     $asm.puts "mov#{x86Suffix(kind)} #{orderOperands(operands[1].x86Operand(kind), operands[2].x86Operand(kind))}"
                 end
@@ -788 +788 @@
     
     def handleX86Sub(kind)
-        if operands.size == 3 and operands[1] == operands[2]
-            $asm.puts "neg#{x86Suffix(kind)} #{operands[2].x86Operand(kind)}"
-            $asm.puts "add#{x86Suffix(kind)} #{orderOperands(operands[0].x86Operand(kind), operands[2].x86Operand(kind))}"
-        else
-            handleX86Op("sub#{x86Suffix(kind)}", kind)
-        end
+        if operands.size == 3
+            if Immediate.new(nil, 0) == operands[1]
+                raise unless operands[0].is_a? RegisterID
+                raise unless operands[2].is_a? RegisterID
+                if operands[0] != operands[2]
+                    $asm.puts "mov#{x86Suffix(kind)} #{orderOperands(operands[0].x86Operand(kind), operands[2].x86Operand(kind))}"
+                end
+                return
+            end
+            if operands[1] == operands[2]
+                $asm.puts "neg#{x86Suffix(kind)} #{operands[2].x86Operand(kind)}"
+                if Immediate.new(nil, 0) != operands[0]
+                    $asm.puts "add#{x86Suffix(kind)} #{orderOperands(operands[0].x86Operand(kind), operands[2].x86Operand(kind))}"
+                end
+                return
+            end
+        end
+
+        if operands.size == 2
+            if Immediate.new(nil, 0) == operands[0]
+                return
+            end
+        end
+
+        handleX86Op("sub#{x86Suffix(kind)}", kind)
     end