[llvm] [SimplifyCFG] Treat umul + extract pattern as cheap single instruction. (PR #124933)

[Gábor Spaits via llvm-commits]

================
@@ -0,0 +1,54 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S < %s | FileCheck %s
+
+define i16 @func2(i64 %x, i64 %y) {
+; CHECK-LABEL: @func2(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP_NOT:%.*]] = icmp eq i64 [[Y:%.*]], 0
+; CHECK-NEXT:    [[MUL:%.*]] = tail call { i64, i1 } @llvm.umul.with.overflow.i64(i64 [[Y]], i64 [[X:%.*]])
+; CHECK-NEXT:    [[MUL_OV:%.*]] = extractvalue { i64, i1 } [[MUL]], 1
+; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[CMP_NOT]], i1 false, i1 [[MUL_OV]]
+; CHECK-NEXT:    [[CONV:%.*]] = zext i1 [[SPEC_SELECT]] to i16
+; CHECK-NEXT:    ret i16 [[CONV]]
+;
+entry:
+  %cmp.not = icmp eq i64 %y, 0
+  br i1 %cmp.not, label %land.end, label %land.rhs
+
+land.rhs:                                         ; preds = %entry
+  %mul = tail call { i64, i1 } @llvm.umul.with.overflow.i64(i64 %y, i64 %x)
+  %mul.ov = extractvalue { i64, i1 } %mul, 1
+  br label %land.end
+
+land.end:                                         ; preds = %land.rhs, %entry
+  %0 = phi i1 [ false, %entry ], [ %mul.ov, %land.rhs ]
+  %conv = zext i1 %0 to i16
+  ret i16 %conv
+}
+
+define i16 @noHoist(i64 %x, i64 %y) {
+; CHECK-LABEL: @noHoist(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP_NOT:%.*]] = icmp eq i64 [[Y:%.*]], 0
+; CHECK-NEXT:    [[ADD2:%.*]] = add nsw i64 [[Y]], [[X:%.*]]
+; CHECK-NEXT:    [[MUL:%.*]] = tail call { i64, i1 } @llvm.umul.with.overflow.i64(i64 [[ADD2]], i64 [[X]])
+; CHECK-NEXT:    [[MUL_OV:%.*]] = extractvalue { i64, i1 } [[MUL]], 1
+; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[CMP_NOT]], i1 false, i1 [[MUL_OV]]
+; CHECK-NEXT:    [[CONV:%.*]] = zext i1 [[SPEC_SELECT]] to i16
+; CHECK-NEXT:    ret i16 [[CONV]]
+;
+entry:
+  %cmp.not = icmp eq i64 %y, 0
+  br i1 %cmp.not, label %land.end, label %land.rhs
+
+land.rhs:                                   ; preds = %entry
+  %add = add nsw i64 %y, %x
----------------
spaits wrote:

@nikic Even if the extra instruction is here, we still hoist.
What is interesting, if I put the following debug prints in the code when instruction count checking:
```cpp
    if (SpeculatedInstructions > MaxSpeculatedInstructionsToHoist) {
      llvm::errs() << "Returning:\n";
      I.dump();
      llvm::errs() << "\n";
      return false;
    }
    llvm::errs() << "Proceed\n";
    I.dump();
    llvm::errs() << "\n";
```
(I know, not the best way to debug :) )
I get this in my terminal:
```
ninja opt && ../llvm/utils/update_test_checks.py --tool-binary=bin/opt ../llvm/test/Transforms/SimplifyCFG/umul-extract-pattern.ll 
ninja: no work to do.
...
Proceed
  %mul.ov = extractvalue { i64, i1 } %mul, 1

Proceed
  %mul = tail call { i64, i1 } @llvm.umul.with.overflow.i64(i64 %add, i64 %x)

Returning:
  %add = add nsw i64 %y, %x
```
So to me, it seems that we run onto the `return false`.
I will investigate this further.

https://github.com/llvm/llvm-project/pull/124933