[llvm] 83d1788 - [InstCombine] Set zero_is_poison for ctlz/cttz if they are only used as shift amounts (#85035)

[via llvm-commits]

Author: Yingwei Zheng
Date: 2024-03-13T21:52:40+08:00
New Revision: 83d178843f4322159b6469f430a8a241b8672d6d

URL: https://github.com/llvm/llvm-project/commit/83d178843f4322159b6469f430a8a241b8672d6d
DIFF: https://github.com/llvm/llvm-project/commit/83d178843f4322159b6469f430a8a241b8672d6d.diff

LOG: [InstCombine] Set zero_is_poison for ctlz/cttz if they are only used as shift amounts (#85035)

Alive2: https://alive2.llvm.org/ce/z/r-67t9

It would improve the codegen if the target doesn't provide a defined
value for ctlz/cttz with zero.

Added: 
    llvm/test/Transforms/InstCombine/shift-cttz-ctlz.ll

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index f5f3716d390d77..694b18017babc5 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -504,6 +504,11 @@ static Instruction *foldCttzCtlz(IntrinsicInst &II, InstCombinerImpl &IC) {
     return IC.replaceInstUsesWith(II, ConstantInt::getNullValue(II.getType()));
   }
 
+  // If ctlz/cttz is only used as a shift amount, set is_zero_poison to true.
+  if (II.hasOneUse() && match(Op1, m_Zero()) &&
+      match(II.user_back(), m_Shift(m_Value(), m_Specific(&II))))
+    return IC.replaceOperand(II, 1, IC.Builder.getTrue());
+
   Constant *C;
 
   if (IsTZ) {

diff  --git a/llvm/test/Transforms/InstCombine/shift-cttz-ctlz.ll b/llvm/test/Transforms/InstCombine/shift-cttz-ctlz.ll
new file mode 100644
index 00000000000000..2b2f820c9a0956
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/shift-cttz-ctlz.ll
@@ -0,0 +1,93 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+
+define i32 @shl_cttz_false(i32 %x, i32 %y) {
+; CHECK-LABEL: define i32 @shl_cttz_false(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CTTZ:%.*]] = call i32 @llvm.cttz.i32(i32 [[Y]], i1 true), !range [[RNG0:![0-9]+]]
+; CHECK-NEXT:    [[RES:%.*]] = shl i32 [[X]], [[CTTZ]]
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+entry:
+  %cttz = call i32 @llvm.cttz.i32(i32 %y, i1 false)
+  %res = shl i32 %x, %cttz
+  ret i32 %res
+}
+
+define i32 @shl_ctlz_false(i32 %x, i32 %y) {
+; CHECK-LABEL: define i32 @shl_ctlz_false(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CTTZ:%.*]] = call i32 @llvm.ctlz.i32(i32 [[Y]], i1 true), !range [[RNG0]]
+; CHECK-NEXT:    [[RES:%.*]] = shl i32 [[X]], [[CTTZ]]
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+entry:
+  %cttz = call i32 @llvm.ctlz.i32(i32 %y, i1 false)
+  %res = shl i32 %x, %cttz
+  ret i32 %res
+}
+
+define i32 @lshr_cttz_false(i32 %x, i32 %y) {
+; CHECK-LABEL: define i32 @lshr_cttz_false(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CTTZ:%.*]] = call i32 @llvm.cttz.i32(i32 [[Y]], i1 true), !range [[RNG0]]
+; CHECK-NEXT:    [[RES:%.*]] = lshr i32 [[X]], [[CTTZ]]
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+entry:
+  %cttz = call i32 @llvm.cttz.i32(i32 %y, i1 false)
+  %res = lshr i32 %x, %cttz
+  ret i32 %res
+}
+
+define i32 @ashr_cttz_false(i32 %x, i32 %y) {
+; CHECK-LABEL: define i32 @ashr_cttz_false(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CTTZ:%.*]] = call i32 @llvm.cttz.i32(i32 [[Y]], i1 true), !range [[RNG0]]
+; CHECK-NEXT:    [[RES:%.*]] = ashr i32 [[X]], [[CTTZ]]
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+entry:
+  %cttz = call i32 @llvm.cttz.i32(i32 %y, i1 false)
+  %res = ashr i32 %x, %cttz
+  ret i32 %res
+}
+
+define i32 @shl_cttz_false_multiuse(i32 %x, i32 %y) {
+; CHECK-LABEL: define i32 @shl_cttz_false_multiuse(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CTTZ:%.*]] = call i32 @llvm.cttz.i32(i32 [[Y]], i1 false), !range [[RNG0]]
+; CHECK-NEXT:    call void @use(i32 [[CTTZ]])
+; CHECK-NEXT:    [[RES:%.*]] = shl i32 [[X]], [[CTTZ]]
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+entry:
+  %cttz = call i32 @llvm.cttz.i32(i32 %y, i1 false)
+  call void @use(i32 %cttz)
+  %res = shl i32 %x, %cttz
+  ret i32 %res
+}
+
+define i32 @shl_cttz_as_lhs(i32 %x, i32 %y) {
+; CHECK-LABEL: define i32 @shl_cttz_as_lhs(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CTTZ:%.*]] = call i32 @llvm.cttz.i32(i32 [[Y]], i1 false), !range [[RNG0]]
+; CHECK-NEXT:    [[RES:%.*]] = shl i32 [[CTTZ]], [[X]]
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+entry:
+  %cttz = call i32 @llvm.cttz.i32(i32 %y, i1 false)
+  %res = shl i32 %cttz, %x
+  ret i32 %res
+}
+
+declare void @use(i32)
+;.
+; CHECK: [[RNG0]] = !{i32 0, i32 33}
+;.