[ConstraintElim] Decompose shl nsw for signed predicates

[nikic]

shl nsw x, shift can be interpreted as mul nsw x, (1<<shift), except when shift is bw-1 (https://alive2.llvm.org/ce/z/vDh2xT). Use this when decomposing shl. The equivalent decomposition for the unsigned case already exists.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Nikita Popov (nikic)

Changes

shl nsw x, shift can be interpreted as mul nsw x, (1<<shift), except when shift is bw-1 (https://alive2.llvm.org/ce/z/vDh2xT). Use this when decomposing shl. The equivalent decomposition for the unsigned case already exists.

---

Full diff: https://github.com/llvm/llvm-project/pull/76961.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Scalar/ConstraintElimination.cpp (+11)
• (modified) llvm/test/Transforms/ConstraintElimination/shl.ll (+3-6)

diff --git a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
index 9a814ba9fd7380..05c56cae4dacfb 100644
--- a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
@@ -517,6 +517,17 @@ static Decomposition decompose(Value *V,
       return Result;
     }
 
+    // shl nsw x, shift is mul nsw x, (1<<shift),
+    // with the exception of shift == bw-1.
+    if (match(V, m_NSWShl(m_Value(Op0), m_ConstantInt(CI)))) {
+      uint64_t Shift = CI->getValue().getLimitedValue();
+      if (Shift < Ty->getIntegerBitWidth() - 1 && Shift < 64) {
+        auto Result = decompose(Op0, Preconditions, IsSigned, DL);
+        Result.mul(int64_t(1) << Shift);
+        return Result;
+      }
+    }
+
     return V;
   }
 
diff --git a/llvm/test/Transforms/ConstraintElimination/shl.ll b/llvm/test/Transforms/ConstraintElimination/shl.ll
index 8a00eb9b2830ba..4af54da1f7e817 100644
--- a/llvm/test/Transforms/ConstraintElimination/shl.ll
+++ b/llvm/test/Transforms/ConstraintElimination/shl.ll
@@ -1283,8 +1283,7 @@ define i1 @shl_nsw_x8_slt_x7(i8 %start, i8 %high) {
 ; CHECK-NEXT:    [[C_1:%.*]] = icmp slt i8 [[START_SHL_3]], [[HIGH]]
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[C_1]])
 ; CHECK-NEXT:    [[START_MUL_7:%.*]] = mul nsw i8 [[START]], 7
-; CHECK-NEXT:    [[T_1:%.*]] = icmp slt i8 [[START_MUL_7]], [[HIGH]]
-; CHECK-NEXT:    ret i1 [[T_1]]
+; CHECK-NEXT:    ret i1 true
 ;
   %c.0 = icmp sge i8 %high, 0
   call void @llvm.assume(i1 %c.0)
@@ -1327,11 +1326,9 @@ define i1 @shl_nsw_sign_implication(i8 %x) {
 ; CHECK-NEXT:    [[CMP1:%.*]] = icmp slt i8 [[X]], 0
 ; CHECK-NEXT:    br i1 [[CMP1]], label [[IF:%.*]], label [[ELSE:%.*]]
 ; CHECK:       if:
-; CHECK-NEXT:    [[CMP2:%.*]] = icmp slt i8 [[SHL]], 0
-; CHECK-NEXT:    ret i1 [[CMP2]]
+; CHECK-NEXT:    ret i1 true
 ; CHECK:       else:
-; CHECK-NEXT:    [[CMP3:%.*]] = icmp sge i8 [[SHL]], 0
-; CHECK-NEXT:    ret i1 [[CMP3]]
+; CHECK-NEXT:    ret i1 true
 ;
   %shl = shl nsw i8 %x, 2
   %cmp1 = icmp slt i8 %x, 0

[fhahn]

LGTM, thanks!

[fhahn]

nit: reflow comment?

[fhahn]

nit: might be good to wrap the expressions in () or something else to separate them from the interleave text

[fhahn]

Looks like there's no test case for Shift >= 64 at the moment. IIUC this is to avoid overflows when doing 1 << Shift? We won't reach this code if the types are wider than 64 bit and if the type is <= i64 then shifts >= 64 will produce poison. Might be helpful for future readers to document the condition

[nikic]

Yeah, this check is actually redundant. I'll replace it with an assert.

[fhahn]

Sounds good!

[DaniilSuchkov]

Hi! This change seems to have caused a miscompile: #78621