[Support] Refactor getN1Bits so it does not work around any g++ bug (llvm#78933)

AZero13 · web-flow · commit bb02bf78946a · 2024-05-26T17:25:02.000-04:00
This also folds better than the previous version as well. Proof: https://alive2.llvm.org/ce/z/6uwy95
diff --git a/llvm/include/llvm/Support/Discriminator.h b/llvm/include/llvm/Support/Discriminator.h
@@ -121,12 +121,9 @@ static inline unsigned getBaseFSBitEnd() {
 }
 
 // Set bits in range of [0 .. n] to 1. Used in FS Discriminators.
-static inline unsigned getN1Bits(int N) {
-  // Work around the g++ bug that folding "(1U << (N + 1)) - 1" to 0.
-  if (N == 31)
-    return 0xFFFFFFFF;
+static inline unsigned getN1Bits(unsigned N) {
   assert((N < 32) && "N is invalid");
-  return (1U << (N + 1)) - 1;
+  return 0xFFFFFFFF >> (31 - N);
 }
 
 } // namespace llvm

Original file line number	Diff line number	Diff line change
`@@ -121,12 +121,9 @@ static inline unsigned getBaseFSBitEnd() {`
`121`	`121`	`}`
`122`	`122`
`123`	`123`	`// Set bits in range of [0 .. n] to 1. Used in FS Discriminators.`
`124`		`-static inline unsigned getN1Bits(int N) {`
`125`		`- // Work around the g++ bug that folding "(1U << (N + 1)) - 1" to 0.`
`126`		`- if (N == 31)`
`127`		`- return 0xFFFFFFFF;`
	`124`	`+static inline unsigned getN1Bits(unsigned N) {`
`128`	`125`	`assert((N < 32) && "N is invalid");`
`129`		`- return (1U << (N + 1)) - 1;`
	`126`	`+ return 0xFFFFFFFF >> (31 - N);`
`130`	`127`	`}`
`131`	`128`
`132`	`129`	`} // namespace llvm`