[llvm] [llvm-exegesis] Add CLI Option to set Fixed RNG seed (PR #170013)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/170013

The primary motivation for this is to set a fixed RNG seed for flaky
tests. This also has the bonus of adding debug logging for what seed
gets used which can make it much easier to reproduce issues that only
happen occasionally and are seed-dependent.


>From ee843445ebc71c427addb8238687d808ec043c75 Mon Sep 17 00:00:00 2001
From: Aiden Grossman <aidengrossman at google.com>
Date: Sat, 29 Nov 2025 23:41:12 +0000
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.7
---
 .../X86/snippet-generator-seed.test              | 16 ++++++++++++++++
 .../tools/llvm-exegesis/lib/SnippetGenerator.cpp | 15 ++++++++++++++-
 2 files changed, 30 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/tools/llvm-exegesis/X86/snippet-generator-seed.test

diff --git a/llvm/test/tools/llvm-exegesis/X86/snippet-generator-seed.test b/llvm/test/tools/llvm-exegesis/X86/snippet-generator-seed.test
new file mode 100644
index 0000000000000..54a0e4946fcd8
--- /dev/null
+++ b/llvm/test/tools/llvm-exegesis/X86/snippet-generator-seed.test
@@ -0,0 +1,16 @@
+# REQUIRES: exegesis-can-measure-latency, x86_64-linux
+
+# Check that the snippet we generate is exactly the same between runs when we
+# use a fixed RNG seed.
+
+# RUN: llvm-exegesis -mtriple=x86_64-unknown-unknown -mode=latency -opcode-name=ADD64rr --benchmark-phase=prepare-snippet -random-generator-seed=5 | FileCheck %s
+
+# CHECK: ---
+# CHECK: mode:            latency
+# CHECK: key:
+# CHECK:   instructions:
+# CHECK:     - 'ADD64rr RCX RCX RAX'
+# CHECK:   config:          ''
+# CHECK:   register_initial_values:
+# CHECK:     - 'RCX=0x0'
+# CHECK:     - 'RAX=0x0'
diff --git a/llvm/tools/llvm-exegesis/lib/SnippetGenerator.cpp b/llvm/tools/llvm-exegesis/lib/SnippetGenerator.cpp
index 7023f1bfae193..17cdf328af84c 100644
--- a/llvm/tools/llvm-exegesis/lib/SnippetGenerator.cpp
+++ b/llvm/tools/llvm-exegesis/lib/SnippetGenerator.cpp
@@ -21,9 +21,17 @@
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/Program.h"
 
+#define DEBUG_TYPE "snippet-generator"
+
 namespace llvm {
 namespace exegesis {
 
+static cl::opt<unsigned>
+    RandomGeneratorSeed("random-generator-seed",
+                        cl::desc("The seed value to use for the random number "
+                                 "generator when generating snippets."),
+                        cl::init(0));
+
 std::vector<CodeTemplate> getSingleton(CodeTemplate &&CT) {
   std::vector<CodeTemplate> Result;
   Result.push_back(std::move(CT));
@@ -187,8 +195,13 @@ generateUnconstrainedCodeTemplates(const InstructionTemplate &Variant,
 }
 
 std::mt19937 &randomGenerator() {
+  unsigned RandomSeed = RandomGeneratorSeed;
   static std::random_device RandomDevice;
-  static std::mt19937 RandomGenerator(RandomDevice());
+  if (RandomSeed == 0) {
+    RandomSeed = RandomDevice();
+  }
+  LLVM_DEBUG(dbgs() << "Using random seed " << RandomSeed << ".\n");
+  static std::mt19937 RandomGenerator(RandomSeed);
   return RandomGenerator;
 }