[PATCH] D141464: [X86]: Match (xor TSize - 1, ctlz) to `bsr` instead of `lzcnt` + `xor`

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n created this revision.
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Was previously de-optimizating if -march supported lzcnt as there is
no reason to add the extra instruction.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D141464

Files:
  llvm/lib/Target/X86/X86InstrCompiler.td
  llvm/test/CodeGen/X86/clz.ll


Index: llvm/test/CodeGen/X86/clz.ll
===================================================================
--- llvm/test/CodeGen/X86/clz.ll
+++ llvm/test/CodeGen/X86/clz.ll
@@ -741,14 +741,12 @@
 ;
 ; X86-CLZ-LABEL: ctlz_bsr:
 ; X86-CLZ:       # %bb.0:
-; X86-CLZ-NEXT:    lzcntl {{[0-9]+}}(%esp), %eax
-; X86-CLZ-NEXT:    xorl $31, %eax
+; X86-CLZ-NEXT:    bsrl {{[0-9]+}}(%esp), %eax
 ; X86-CLZ-NEXT:    retl
 ;
 ; X64-CLZ-LABEL: ctlz_bsr:
 ; X64-CLZ:       # %bb.0:
-; X64-CLZ-NEXT:    lzcntl %edi, %eax
-; X64-CLZ-NEXT:    xorl $31, %eax
+; X64-CLZ-NEXT:    bsrl %edi, %eax
 ; X64-CLZ-NEXT:    retq
   %ctlz = call i32 @llvm.ctlz.i32(i32 %n, i1 true)
   %bsr = xor i32 %ctlz, 31
@@ -790,14 +788,12 @@
 ;
 ; X86-CLZ-LABEL: ctlz_bsr_zero_test:
 ; X86-CLZ:       # %bb.0:
-; X86-CLZ-NEXT:    lzcntl {{[0-9]+}}(%esp), %eax
-; X86-CLZ-NEXT:    xorl $31, %eax
+; X86-CLZ-NEXT:    bsrl {{[0-9]+}}(%esp), %eax
 ; X86-CLZ-NEXT:    retl
 ;
 ; X64-CLZ-LABEL: ctlz_bsr_zero_test:
 ; X64-CLZ:       # %bb.0:
-; X64-CLZ-NEXT:    lzcntl %edi, %eax
-; X64-CLZ-NEXT:    xorl $31, %eax
+; X64-CLZ-NEXT:    bsrl %edi, %eax
 ; X64-CLZ-NEXT:    retq
   %ctlz = call i32 @llvm.ctlz.i32(i32 %n, i1 false)
   %bsr = xor i32 %ctlz, 31
@@ -1079,8 +1075,7 @@
 ; X86-CLZ-LABEL: PR47603_zext:
 ; X86-CLZ:       # %bb.0:
 ; X86-CLZ-NEXT:    movl {{[0-9]+}}(%esp), %eax
-; X86-CLZ-NEXT:    lzcntl {{[0-9]+}}(%esp), %ecx
-; X86-CLZ-NEXT:    xorl $31, %ecx
+; X86-CLZ-NEXT:    bsrl {{[0-9]+}}(%esp), %ecx
 ; X86-CLZ-NEXT:    movsbl (%eax,%ecx), %eax
 ; X86-CLZ-NEXT:    retl
 ;
Index: llvm/lib/Target/X86/X86InstrCompiler.td
===================================================================
--- llvm/lib/Target/X86/X86InstrCompiler.td
+++ llvm/lib/Target/X86/X86InstrCompiler.td
@@ -2247,3 +2247,13 @@
 let Predicates = [HasMOVBE] in {
  def : Pat<(bswap GR16:$src), (ROL16ri GR16:$src, (i8 8))>;
 }
+
+
+// Match idiomatic (sizeof(x) - 1 - clz(x)) with bsr instead of
+// emitting extra xor + lzcnt.
+def : Pat<(xor (ctlz GR16:$src), 15), (BSR16rr GR16:$src)>;
+def : Pat<(xor (ctlz GR32:$src), 31), (BSR32rr GR32:$src)>;
+def : Pat<(xor (ctlz GR64:$src), 63), (BSR64rr GR64:$src)>;
+def : Pat<(xor (ctlz_zero_undef GR16:$src), 15), (BSR16rr GR16:$src)>;
+def : Pat<(xor (ctlz_zero_undef GR32:$src), 31), (BSR32rr GR32:$src)>;
+def : Pat<(xor (ctlz_zero_undef GR64:$src), 63), (BSR64rr GR64:$src)>;


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