[InstCombine] Added optimisation for trunc (Negated Pow2 >> x) to i1

[kper]

Follow up of #157030

trunc ( lshr i8 C1, V1) to i1 -> icmp ugt V1, cttz(C1) - 1 iff (C1) is negative power of 2
trunc ( ashr i8 C1, V1) to i1 -> icmp ugt V1, cttz(C1) - 1 iff (C1) is negative power of 2

General proof
lshr: https://alive2.llvm.org/ce/z/vVfaJc
ashr: https://alive2.llvm.org/ce/z/8aAcgD

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: None (kper)

Changes

Follow up of #157030

trunc ( lshr i8 C1, V1) to i1 -> icmp ugt V1, cttz(C1) - 1 iff (C1) is negative power of 2
trunc ( ashr i8 C1, V1) to i1 -> icmp ugt V1, cttz(C1) - 1 iff (C1) is negative power of 2

General proof
lshr: https://alive2.llvm.org/ce/z/F5C4Lj
ashr: https://alive2.llvm.org/ce/z/E2bLN7

---

Full diff: https://github.com/llvm/llvm-project/pull/157998.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp (+9)
• (modified) llvm/test/Transforms/InstCombine/trunc-lshr.ll (+74)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index ccf918f0b6dbe..839286ca83ce9 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -990,6 +990,15 @@ Instruction *InstCombinerImpl::visitTrunc(TruncInst &Trunc) {
     return replaceInstUsesWith(Trunc, Icmp);
   }
 
+  // OP = { lshr, ashr }
+  // trunc ( OP i8 C1, V1) to i1 -> icmp ugt V1, cttz(C1) - 1 iff (C1) is
+  // negative power of 2
+  if (DestWidth == 1 && match(Src, m_Shr(m_NegatedPower2(C1), m_Value(V1)))) {
+    Value *Right = ConstantInt::get(V1->getType(), C1->countr_zero() - 1);
+    Value *Icmp = Builder.CreateICmpUGT(V1, Right);
+    return replaceInstUsesWith(Trunc, Icmp);
+  }
+
   return Changed ? &Trunc : nullptr;
 }
 
diff --git a/llvm/test/Transforms/InstCombine/trunc-lshr.ll b/llvm/test/Transforms/InstCombine/trunc-lshr.ll
index c443b35cb1c1e..0e996e5d017fe 100644
--- a/llvm/test/Transforms/InstCombine/trunc-lshr.ll
+++ b/llvm/test/Transforms/InstCombine/trunc-lshr.ll
@@ -219,3 +219,77 @@ define i1 @negative_test_fold_ashr(i8 %x) {
   %trunc = trunc i8 %ashr to i1
   ret i1 %trunc
 }
+
+define i1 @fold_lshr_negated_power_of_2(i8 %x) {
+; CHECK-LABEL: define i1 @fold_lshr_negated_power_of_2(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT:    [[TRUNC:%.*]] = icmp ugt i8 [[X]], 3
+; CHECK-NEXT:    ret i1 [[TRUNC]]
+;
+  %lshr = lshr i8 -16, %x
+  %trunc = trunc i8 %lshr to i1
+  ret i1 %trunc
+}
+
+define i1 @fold_ashr_negated_power_of_2(i8 %x) {
+; CHECK-LABEL: define i1 @fold_ashr_negated_power_of_2(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT:    [[TRUNC:%.*]] = icmp ugt i8 [[X]], 3
+; CHECK-NEXT:    ret i1 [[TRUNC]]
+;
+  %ashr = ashr i8 -16, %x
+  %trunc = trunc i8 %ashr to i1
+  ret i1 %trunc
+}
+
+define i1 @fold_lshr_negated_power_of_2_multi_use(i8 %x) {
+; CHECK-LABEL: define i1 @fold_lshr_negated_power_of_2_multi_use(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT:    [[LSHR:%.*]] = lshr i8 -16, [[X]]
+; CHECK-NEXT:    call void @use(i8 [[LSHR]])
+; CHECK-NEXT:    [[TRUNC:%.*]] = icmp ugt i8 [[X]], 3
+; CHECK-NEXT:    ret i1 [[TRUNC]]
+;
+  %lshr = lshr i8 -16, %x
+  call void @use(i8 %lshr)
+  %trunc = trunc i8 %lshr to i1
+  ret i1 %trunc
+}
+
+define i1 @fold_ashr_negated_power_of_2_multi_use(i8 %x) {
+; CHECK-LABEL: define i1 @fold_ashr_negated_power_of_2_multi_use(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT:    [[ASHR:%.*]] = ashr i8 -16, [[X]]
+; CHECK-NEXT:    call void @use(i8 [[ASHR]])
+; CHECK-NEXT:    [[TRUNC:%.*]] = icmp ugt i8 [[X]], 3
+; CHECK-NEXT:    ret i1 [[TRUNC]]
+;
+  %ashr = ashr i8 -16, %x
+  call void @use(i8 %ashr)
+  %trunc = trunc i8 %ashr to i1
+  ret i1 %trunc
+}
+
+define i1 @negative_test_fold_lshr_negated_power_of_2(i8 %x) {
+; CHECK-LABEL: define i1 @negative_test_fold_lshr_negated_power_of_2(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT:    [[LSHR:%.*]] = lshr i8 -17, [[X]]
+; CHECK-NEXT:    [[TRUNC:%.*]] = trunc i8 [[LSHR]] to i1
+; CHECK-NEXT:    ret i1 [[TRUNC]]
+;
+  %lshr = lshr i8 -17, %x
+  %trunc = trunc i8 %lshr to i1
+  ret i1 %trunc
+}
+
+define i1 @negative_test_fold_ashr_negated_power_of_2(i8 %x) {
+; CHECK-LABEL: define i1 @negative_test_fold_ashr_negated_power_of_2(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT:    [[ASHR1:%.*]] = lshr i8 -17, [[X]]
+; CHECK-NEXT:    [[TRUNC:%.*]] = trunc i8 [[ASHR1]] to i1
+; CHECK-NEXT:    ret i1 [[TRUNC]]
+;
+  %ashr = ashr i8 -17, %x
+  %trunc = trunc i8 %ashr to i1
+  ret i1 %trunc
+}

[kper]

@dtcxzyw can you run the benchmark to see whether it has some real world improvements?

[nikic]

General proof lshr: https://alive2.llvm.org/ce/z/F5C4Lj
ashr: https://alive2.llvm.org/ce/z/E2bLN7

These proofs aren't correct: They check for a power of two which is a negative number, which is not the same as a negated power of two.

Correct ones should be:

https://alive2.llvm.org/ce/z/vVfaJc
https://alive2.llvm.org/ce/z/8aAcgD

Note that these use uge without the -1, otherwise the transform is incorrect for the case where the negative power of two is -1.

[nikic]

LGTM

[nikic]

@zyw-bot mfuzz

[dtcxzyw]

Suggested change

	Value *Icmp = Builder.CreateICmpUGE(V1, Right);
	return replaceInstUsesWith(Trunc, Icmp);
	return new ICmpInst(ICmpInst::ICMP_UGE, Icmp, Right);

See https://llvm.org/docs/InstCombineContributorGuide.html#instcombine-apis.
Can you also modify replaceInstUsesWith above (introduced by #157030)?