[PATCH] D121147: [x86] try harder to use shift instead of test if it can save some immediate bytes

[Sanjay Patel via Phabricator via llvm-commits]

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We favor 'and' and later 'test' in earlier phases, and that's usually the better option, but we can save a few instruction bytes by converting a mask constant to a shift here.


https://reviews.llvm.org/D121147

Files:
  llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
  llvm/test/CodeGen/X86/cmp.ll


Index: llvm/test/CodeGen/X86/cmp.ll
===================================================================
--- llvm/test/CodeGen/X86/cmp.ll
+++ llvm/test/CodeGen/X86/cmp.ll
@@ -413,8 +413,7 @@
 ; CHECK-LABEL: highmask_i64_mask32:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    xorl %eax, %eax # encoding: [0x31,0xc0]
-; CHECK-NEXT:    testq $-1048576, %rdi # encoding: [0x48,0xf7,0xc7,0x00,0x00,0xf0,0xff]
-; CHECK-NEXT:    # imm = 0xFFF00000
+; CHECK-NEXT:    shrq $20, %rdi # encoding: [0x48,0xc1,0xef,0x14]
 ; CHECK-NEXT:    sete %al # encoding: [0x0f,0x94,0xc0]
 ; CHECK-NEXT:    retq # encoding: [0xc3]
   %and = and i64 %val, -1048576
@@ -453,8 +452,7 @@
 ; CHECK-LABEL: lowmask_i64_mask32:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    xorl %eax, %eax # encoding: [0x31,0xc0]
-; CHECK-NEXT:    testl $1048575, %edi # encoding: [0xf7,0xc7,0xff,0xff,0x0f,0x00]
-; CHECK-NEXT:    # imm = 0xFFFFF
+; CHECK-NEXT:    shlq $44, %rdi # encoding: [0x48,0xc1,0xe7,0x2c]
 ; CHECK-NEXT:    setne %al # encoding: [0x0f,0x95,0xc0]
 ; CHECK-NEXT:    retq # encoding: [0xc3]
   %and = and i64 %val, 1048575
Index: llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
===================================================================
--- llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
+++ llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -5616,9 +5616,11 @@
 
       // Check if we can replace AND+IMM64 with a shift. This is possible for
       // masks like 0xFF000000 or 0x00FFFFFF and if we care only about the zero
-      // flag.
-      if (CmpVT == MVT::i64 && !isInt<32>(Mask) &&
-          onlyUsesZeroFlag(SDValue(Node, 0))) {
+      // flag. Also, test the operand of the 'and' for extra uses because that
+      // may require a register copy when converting to a shift.
+      // TODO: Extend this to handle i32.
+      if (N0.getOperand(0).hasOneUse() && CmpVT == MVT::i64 &&
+          !isInt<8>(Mask) && onlyUsesZeroFlag(SDValue(Node, 0))) {
         unsigned ShiftOpcode = ISD::DELETED_NODE;
         unsigned ShiftAmt;
         if (isMask_64(~Mask)) {


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