[PATCH] D146787: [X86] Teach computeKnownBitsForTargetNode about MUL_IMM

[Kazu Hirata via Phabricator via llvm-commits]

kazu updated this revision to Diff 507979.
kazu added a comment.

Guard against a potential integer overflow.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D146787/new/

https://reviews.llvm.org/D146787

Files:
  llvm/lib/Target/X86/X86ISelLowering.cpp
  llvm/test/CodeGen/X86/select-constant-lea.ll


Index: llvm/test/CodeGen/X86/select-constant-lea.ll
===================================================================
--- llvm/test/CodeGen/X86/select-constant-lea.ll
+++ llvm/test/CodeGen/X86/select-constant-lea.ll
@@ -8,8 +8,7 @@
 ; BASE-NEXT:    xorl %eax, %eax
 ; BASE-NEXT:    cmpl $10, %edi
 ; BASE-NEXT:    setae %al
-; BASE-NEXT:    leal (%rax,%rax,4), %eax
-; BASE-NEXT:    orl $8, %eax
+; BASE-NEXT:    leal 8(%rax,%rax,4), %eax
 ; BASE-NEXT:    retq
 ;
 ; SLOWLEA3-LABEL: select_unsigned_lt_10_8_13j:
@@ -18,7 +17,7 @@
 ; SLOWLEA3-NEXT:    cmpl $10, %edi
 ; SLOWLEA3-NEXT:    setae %al
 ; SLOWLEA3-NEXT:    leal (%rax,%rax,4), %eax
-; SLOWLEA3-NEXT:    orl $8, %eax
+; SLOWLEA3-NEXT:    addl $8, %eax
 ; SLOWLEA3-NEXT:    retq
   %2 = icmp ult i32 %0, 10
   %3 = select i1 %2, i32 8, i32 13
Index: llvm/lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- llvm/lib/Target/X86/X86ISelLowering.cpp
+++ llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -38098,6 +38098,19 @@
   Known.resetAll();
   switch (Opc) {
   default: break;
+  case X86ISD::MUL_IMM: {
+    KnownBits Known2 =
+        DAG.computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
+    const APInt &Imm = cast<ConstantSDNode>(Op.getOperand(1))->getAPIntValue();
+    APInt Val = Known2.Zero;
+    Val.flipAllBits();
+    // Compute the known zero bits if we don't have to worry about overflow.
+    if (Imm.getActiveBits() + Val.getActiveBits() <= BitWidth) {
+      Val *= Imm;
+      Known.Zero.setBitsFrom(Val.getActiveBits());
+    }
+    break;
+  }
   case X86ISD::SETCC:
     Known.Zero.setBitsFrom(1);
     break;


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