[llvm] e1c808b - Fix zero-width bitfield extracts to emit 0

[Jon Roelofs via llvm-commits]

Author: Jon Roelofs
Date: 2022-05-03T14:46:42-07:00
New Revision: e1c808b36e2ec7050016c2eaafab3e17696583a0

URL: https://github.com/llvm/llvm-project/commit/e1c808b36e2ec7050016c2eaafab3e17696583a0
DIFF: https://github.com/llvm/llvm-project/commit/e1c808b36e2ec7050016c2eaafab3e17696583a0.diff

LOG: Fix zero-width bitfield extracts to emit 0

Fixes #55129

Added: 
    

Modified: 
    llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
    llvm/test/CodeGen/AArch64/GlobalISel/form-bitfield-extract-from-shr-and.mir

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
index 674557b1caedf..cdd85d4ba84ea 100644
--- a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -4384,6 +4384,14 @@ bool CombinerHelper::matchBitfieldExtractFromShrAnd(
   if (ShrAmt < 0 || ShrAmt >= Size)
     return false;
 
+  // If the shift subsumes the mask, emit the 0 directly.
+  if (0 == (SMask >> ShrAmt)) {
+    MatchInfo = [=](MachineIRBuilder &B) {
+      B.buildConstant(Dst, 0);
+    };
+    return true;
+  }
+
   // Check that ubfx can do the extraction, with no holes in the mask.
   uint64_t UMask = SMask;
   UMask |= maskTrailingOnes<uint64_t>(ShrAmt);

diff  --git a/llvm/test/CodeGen/AArch64/GlobalISel/form-bitfield-extract-from-shr-and.mir b/llvm/test/CodeGen/AArch64/GlobalISel/form-bitfield-extract-from-shr-and.mir
index 55a75b1d03fda..c8c5b157f4244 100644
--- a/llvm/test/CodeGen/AArch64/GlobalISel/form-bitfield-extract-from-shr-and.mir
+++ b/llvm/test/CodeGen/AArch64/GlobalISel/form-bitfield-extract-from-shr-and.mir
@@ -188,3 +188,20 @@ body:             |
     %4:_(s32) = G_LSHR %3, %2
     $w0 = COPY %4(s32)
 ...
+---
+name:          zero_from_large_shift
+legalized: true
+body:              |
+  bb.1.entry:
+    ; CHECK-LABEL: name: zero_from_large_shift
+    ; CHECK: [[C:%[0-9]+]]:_(s64) = G_CONSTANT i64 0
+    ; CHECK-NEXT: $x0 = COPY [[C]](s64)
+    %2:_(s32) = COPY $w0
+    %1:_(s8) = G_TRUNC %2:_(s32)
+    %3:_(s8) = G_ASSERT_ZEXT %1:_, 1
+    %5:_(s64) = G_CONSTANT i64 1
+    %7:_(s64) = G_ANYEXT %3:_(s8)
+    %4:_(s64) = G_AND %7:_, %5:_
+    %6:_(s64) = G_LSHR %4:_, %5:_(s64)
+    $x0 = COPY %6:_(s64)
+...