Fix for AMDGPU MUL_I24 known bits calculation

Summary:
At present, the code calculating known bits of AMDGPU MUL_I24 confuses the concepts of "non-negative number" and "positive number".

In some situations, it results in incorrect code. I have a case where the optimizer replaces the result of calculating MUL_I24(-5, 0) with -8.

Reviewers: foad, arsenm

Reviewed By: arsenm

Subscribers: foad, arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Patch by Eugene Kuznetsov.

Differential Revision: https://reviews.llvm.org/D70367

diff --git a/llvm/include/llvm/Support/KnownBits.h b/llvm/include/llvm/Support/KnownBits.h
index df0b02c..ff25b6f 100644 (file)
--- a/llvm/include/llvm/Support/KnownBits.h
+++ b/llvm/include/llvm/Support/KnownBits.h
@@ -97,6 +97,9 @@ public:
   /// Returns true if this value is known to be non-negative.
   bool isNonNegative() const { return Zero.isSignBitSet(); }
 
+  /// Returns true if this value is known to be positive.
+  bool isStrictlyPositive() const { return Zero.isSignBitSet() && !One.isNullValue(); }
+
   /// Make this value negative.
   void makeNegative() {
     One.setSignBit();

diff --git a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
index b250087..8a7b4be 100644 (file)
--- a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
@@ -4445,7 +4445,6 @@ void AMDGPUTargetLowering::computeKnownBitsForTargetNode(
     LHSKnown = LHSKnown.trunc(24);
     RHSKnown = RHSKnown.trunc(24);
 
-    bool Negative = false;
     if (Opc == AMDGPUISD::MUL_I24) {
       unsigned LHSValBits = 24 - LHSKnown.countMinSignBits();
       unsigned RHSValBits = 24 - RHSKnown.countMinSignBits();
@@ -4453,16 +4452,16 @@ void AMDGPUTargetLowering::computeKnownBitsForTargetNode(
       if (MaxValBits >= 32)
         break;
       bool LHSNegative = LHSKnown.isNegative();
-      bool LHSPositive = LHSKnown.isNonNegative();
+      bool LHSNonNegative = LHSKnown.isNonNegative();
+      bool LHSPositive = LHSKnown.isStrictlyPositive();
       bool RHSNegative = RHSKnown.isNegative();
-      bool RHSPositive = RHSKnown.isNonNegative();
-      if ((!LHSNegative && !LHSPositive) || (!RHSNegative && !RHSPositive))
-        break;
-      Negative = (LHSNegative && RHSPositive) || (LHSPositive && RHSNegative);
-      if (Negative)
-        Known.One.setHighBits(32 - MaxValBits);
-      else
+      bool RHSNonNegative = RHSKnown.isNonNegative();
+      bool RHSPositive = RHSKnown.isStrictlyPositive();
+
+      if((LHSNonNegative && RHSNonNegative) || (LHSNegative && RHSNegative))
         Known.Zero.setHighBits(32 - MaxValBits);
+      else if((LHSNegative && RHSPositive) || (LHSPositive && RHSNegative))
+        Known.One.setHighBits(32 - MaxValBits);
     } else {
       unsigned LHSValBits = 24 - LHSKnown.countMinLeadingZeros();
       unsigned RHSValBits = 24 - RHSKnown.countMinLeadingZeros();

diff --git a/llvm/test/CodeGen/AMDGPU/amdgpu-mul24-knownbits.ll b/llvm/test/CodeGen/AMDGPU/amdgpu-mul24-knownbits.ll
new file mode 100644 (file)
index 0000000..1f660df
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/amdgpu-mul24-knownbits.ll
@@ -0,0 +1,34 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+
+; RUN: llc -mtriple amdgcn-amd-amdhsa -mcpu=gfx900 < %s | FileCheck --check-prefix=GCN %s
+define weak_odr amdgpu_kernel void @test_mul24_knownbits_kernel(float addrspace(1)* %p) #4 {
+; GCN-LABEL: test_mul24_knownbits_kernel:
+; GCN:       ; %bb.0: ; %entry
+; GCN-NEXT:    v_and_b32_e32 v0, 3, v0
+; GCN-NEXT:    v_mul_i32_i24_e32 v0, 0xfffffb, v0
+; GCN-NEXT:    s_load_dwordx2 s[0:1], s[4:5], 0x0
+; GCN-NEXT:    v_and_b32_e32 v0, 0xffffffe0, v0
+; GCN-NEXT:    v_ashrrev_i32_e32 v1, 31, v0
+; GCN-NEXT:    v_lshlrev_b64 v[0:1], 2, v[0:1]
+; GCN-NEXT:    s_waitcnt lgkmcnt(0)
+; GCN-NEXT:    v_mov_b32_e32 v2, s1
+; GCN-NEXT:    v_add_co_u32_e32 v0, vcc, s0, v0
+; GCN-NEXT:    v_addc_co_u32_e32 v1, vcc, v2, v1, vcc
+; GCN-NEXT:    v_mov_b32_e32 v2, 0
+; GCN-NEXT:    global_store_dword v[0:1], v2, off
+; GCN-NEXT:    s_endpgm
+entry:
+  %0 = tail call i32 @llvm.amdgcn.workitem.id.x() #28, !range !4
+  %tid = and i32 %0, 3
+  %1 = mul nsw i32 %tid, -5
+  %v1 = and i32 %1, -32
+  %v2 = sext i32 %v1 to i64
+  %v3 = getelementptr inbounds float, float addrspace(1)* %p, i64 %v2
+  store float 0.000, float addrspace(1)* %v3, align 4
+  ret void
+}
+
+; Function Attrs: nounwind readnone speculatable
+declare i32 @llvm.amdgcn.workitem.id.x() #20
+
+!4 = !{i32 0, i32 1024}