[PATCH] D26096: [ValueTracking] recognize obfuscated variants of umin/umax

[Sanjay Patel via llvm-commits]

spatel created this revision.
spatel added reviewers: efriedma, majnemer, sanjoy.
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Herald added a subscriber: mcrosier.

The smallest tests that I could find to expose this are codegen tests (because SelectionDAGBuilder::visitSelect() uses matchSelectPattern to create UMAX/UMIN nodes), so I've semi-arbitrarily chosen x86 as the target for the regression tests.

If these were written as unsigned compares in IR, InstCombine canonicalizes the unsigned compares to signed compares. Ie, running the optimizer pessimizes the codegen for this case on trunk:

  define <4 x i32> @umax_vec(<4 x i32> %x) {
    %cmp = icmp ugt <4 x i32> %x, <i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647>
    %sel = select <4 x i1> %cmp, <4 x i32> %x, <4 x i32> <i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647>
    ret <4 x i32> %sel
  }

$ ./opt  umax.ll -S | ./llc -o - -mattr=avx

  vpmaxud	LCPI0_0(%rip), %xmm0, %xmm0

$ ./opt -instcombine umax.ll -S | ./llc -o - -mattr=avx

  vpxor	%xmm1, %xmm1, %xmm1
  vpcmpgtd	%xmm0, %xmm1, %xmm1
  vmovaps	LCPI0_0(%rip), %xmm2    ## xmm2 = [2147483647,2147483647,2147483647,2147483647]
  vblendvps	%xmm1, %xmm0, %xmm2, %xmm0


https://reviews.llvm.org/D26096

Files:
  lib/Analysis/ValueTracking.cpp
  test/CodeGen/X86/vec_umin_umax.ll


Index: test/CodeGen/X86/vec_umin_umax.ll
===================================================================
--- test/CodeGen/X86/vec_umin_umax.ll
+++ test/CodeGen/X86/vec_umin_umax.ll
@@ -1,15 +1,10 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx | FileCheck %s
 
-; FIXME: These are unsigned min/max ops.
-
 define <4 x i32> @umax_vec1(<4 x i32> %x) {
 ; CHECK-LABEL: umax_vec1:
 ; CHECK:       # BB#0:
-; CHECK-NEXT:    vpxor %xmm1, %xmm1, %xmm1
-; CHECK-NEXT:    vpcmpgtd %xmm0, %xmm1, %xmm1
-; CHECK-NEXT:    vmovaps {{.*#+}} xmm2 = [2147483647,2147483647,2147483647,2147483647]
-; CHECK-NEXT:    vblendvps %xmm1, %xmm0, %xmm2, %xmm0
+; CHECK-NEXT:    vpmaxud {{.*}}(%rip), %xmm0, %xmm0
 ; CHECK-NEXT:    retq
 ;
   %cmp = icmp slt <4 x i32> %x, zeroinitializer
@@ -20,9 +15,7 @@
 define <4 x i32> @umax_vec2(<4 x i32> %x) {
 ; CHECK-LABEL: umax_vec2:
 ; CHECK:       # BB#0:
-; CHECK-NEXT:    vpcmpeqd %xmm1, %xmm1, %xmm1
-; CHECK-NEXT:    vpcmpgtd %xmm1, %xmm0, %xmm1
-; CHECK-NEXT:    vblendvps %xmm1, {{.*}}(%rip), %xmm0, %xmm0
+; CHECK-NEXT:    vpmaxud {{.*}}(%rip), %xmm0, %xmm0
 ; CHECK-NEXT:    retq
 ;
   %cmp = icmp sgt <4 x i32> %x, <i32 -1, i32 -1, i32 -1, i32 -1>
@@ -33,9 +26,7 @@
 define <4 x i32> @umin_vec1(<4 x i32> %x) {
 ; CHECK-LABEL: umin_vec1:
 ; CHECK:       # BB#0:
-; CHECK-NEXT:    vpxor %xmm1, %xmm1, %xmm1
-; CHECK-NEXT:    vpcmpgtd %xmm0, %xmm1, %xmm1
-; CHECK-NEXT:    vblendvps %xmm1, {{.*}}(%rip), %xmm0, %xmm0
+; CHECK-NEXT:    vpminud {{.*}}(%rip), %xmm0, %xmm0
 ; CHECK-NEXT:    retq
 ;
   %cmp = icmp slt <4 x i32> %x, zeroinitializer
@@ -46,10 +37,7 @@
 define <4 x i32> @umin_vec2(<4 x i32> %x) {
 ; CHECK-LABEL: umin_vec2:
 ; CHECK:       # BB#0:
-; CHECK-NEXT:    vpcmpeqd %xmm1, %xmm1, %xmm1
-; CHECK-NEXT:    vpcmpgtd %xmm1, %xmm0, %xmm1
-; CHECK-NEXT:    vmovaps {{.*#+}} xmm2 = [2147483648,2147483648,2147483648,2147483648]
-; CHECK-NEXT:    vblendvps %xmm1, %xmm0, %xmm2, %xmm0
+; CHECK-NEXT:    vpminud {{.*}}(%rip), %xmm0, %xmm0
 ; CHECK-NEXT:    retq
 ;
   %cmp = icmp sgt <4 x i32> %x, <i32 -1, i32 -1, i32 -1, i32 -1>
Index: lib/Analysis/ValueTracking.cpp
===================================================================
--- lib/Analysis/ValueTracking.cpp
+++ lib/Analysis/ValueTracking.cpp
@@ -3981,6 +3981,29 @@
         return {SPF_SMIN, SPNB_NA, false};
       }
     }
+
+    // An unsigned min/max can be written with a signed compare.
+    if ((CmpLHS == TrueVal && match(FalseVal, m_APInt(C2))) ||
+        (CmpLHS == FalseVal && match(TrueVal, m_APInt(C2)))) {
+      // Is the sign bit set?
+      // (X <s 0) ? X : MAXVAL ==> (X >u MAXVAL) ? X : MAXVAL ==> UMAX
+      // (X <s 0) ? MAXVAL : X ==> (X >u MAXVAL) ? MAXVAL : X ==> UMIN
+      if (Pred == CmpInst::ICMP_SLT && *C1 == 0 && C2->isMaxSignedValue()) {
+        LHS = TrueVal;
+        RHS = FalseVal;
+        return {TrueVal == CmpLHS ? SPF_UMAX : SPF_UMIN, SPNB_NA, false};
+      }
+
+      // Is the sign bit clear?
+      // (X >s -1) ? X : MINVAL ==> (X <u MINVAL) ? X : MINVAL ==> UMIN
+      // (X >s -1) ? MINVAL : X ==> (X <u MINVAL) ? MINVAL : X ==> UMAX
+      if (Pred == CmpInst::ICMP_SGT && C1->isAllOnesValue() &&
+          C2->isMinSignedValue()) {
+        LHS = TrueVal;
+        RHS = FalseVal;
+        return {TrueVal == CmpLHS ? SPF_UMIN : SPF_UMAX, SPNB_NA, false};
+      }
+    }
   }
 
   // TODO: (X > 4) ? X : 5   -->  (X >= 5) ? X : 5  -->  MAX(X, 5)


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