[llvm] b723d5a - [llvm-exegesis][x86] Add option to prevent use of xmm8-xmm15 upper SSE registers
Simon Pilgrim via llvm-commits
llvm-commits at lists.llvm.org
Wed Dec 7 09:54:27 PST 2022
Author: Simon Pilgrim
Date: 2022-12-07T17:54:09Z
New Revision: b723d5a625c0135a8c6a72572ba91ddf566e8723
URL: https://github.com/llvm/llvm-project/commit/b723d5a625c0135a8c6a72572ba91ddf566e8723
DIFF: https://github.com/llvm/llvm-project/commit/b723d5a625c0135a8c6a72572ba91ddf566e8723.diff
LOG: [llvm-exegesis][x86] Add option to prevent use of xmm8-xmm15 upper SSE registers
Noticed while trying to use llvm-exegesis to get some accurate capture numbers on some old Atom/Silverment hardware as part of the work with D103695.
These targets' frontends are particularly poor and the use of the xmm8-xmm15 SSE registers results in longer instruction encodings which were affecting the latency/throughput estimates.
Thanks to @lebedev.ri for the --skip-measurements command line argument which made testing much easier!
Differential Revision: https://reviews.llvm.org/D138832
Added:
llvm/test/tools/llvm-exegesis/X86/inverse_throughput/inverse_throughput-disable-upper-sse-registers.s
Modified:
llvm/docs/CommandGuide/llvm-exegesis.rst
llvm/tools/llvm-exegesis/lib/X86/Target.cpp
Removed:
################################################################################
diff --git a/llvm/docs/CommandGuide/llvm-exegesis.rst b/llvm/docs/CommandGuide/llvm-exegesis.rst
index d8f98aa5c3969..1d78f455d1e63 100644
--- a/llvm/docs/CommandGuide/llvm-exegesis.rst
+++ b/llvm/docs/CommandGuide/llvm-exegesis.rst
@@ -213,6 +213,15 @@ OPTIONS
could occur if the sampling is too frequent. A prime number should be used to
avoid consistently skipping certain blocks.
+.. option:: -x86-disable-upper-sse-registers
+
+ Using the upper xmm registers (xmm8-xmm15) forces a longer instruction encoding
+ which may put greater pressure on the frontend fetch and decode stages,
+ potentially reducing the rate that instructions are dispatched to the backend,
+ particularly on older hardware. Comparing baseline results with this mode
+ enabled can help determine the effects of the frontend and can be used to
+ improve latency and throughput estimates.
+
.. option:: -repetition-mode=[duplicate|loop|min]
Specify the repetition mode. `duplicate` will create a large, straight line
diff --git a/llvm/test/tools/llvm-exegesis/X86/inverse_throughput/inverse_throughput-disable-upper-sse-registers.s b/llvm/test/tools/llvm-exegesis/X86/inverse_throughput/inverse_throughput-disable-upper-sse-registers.s
new file mode 100644
index 0000000000000..65921fded8867
--- /dev/null
+++ b/llvm/test/tools/llvm-exegesis/X86/inverse_throughput/inverse_throughput-disable-upper-sse-registers.s
@@ -0,0 +1,10 @@
+# RUN: llvm-exegesis -mtriple=x86_64-unknown-unknown -mcpu=x86-64 -mode=inverse_throughput --skip-measurements -x86-disable-upper-sse-registers -opcode-name=ADDPSrr -repetition-mode=loop | FileCheck %s
+
+CHECK: ---
+CHECK-NEXT: mode: inverse_throughput
+CHECK-NEXT: key:
+CHECK-NEXT: instructions:
+CHECK-NEXT: - 'ADDPSrr [[LHS0:XMM[0-7]]] [[LHS0]] [[RHS0:XMM[0-7]]]'
+CHECK-NEXT: - 'ADDPSrr [[LHS1:XMM[0-7]]] [[LHS1]] [[RHS1:XMM[0-7]]]'
+CHECK-NEXT: - 'ADDPSrr [[LHS2:XMM[0-7]]] [[LHS2]] [[RHS2:XMM[0-7]]]'
+CHECK-NEXT: - 'ADDPSrr [[LHS3:XMM[0-7]]] [[LHS3]] [[RHS3:XMM[0-7]]]'
diff --git a/llvm/tools/llvm-exegesis/lib/X86/Target.cpp b/llvm/tools/llvm-exegesis/lib/X86/Target.cpp
index 6ecb6b17f2071..8ab6a67662118 100644
--- a/llvm/tools/llvm-exegesis/lib/X86/Target.cpp
+++ b/llvm/tools/llvm-exegesis/lib/X86/Target.cpp
@@ -54,6 +54,11 @@ static cl::opt<unsigned> LbrSamplingPeriod(
cl::desc("The sample period (nbranches/sample), used for LBR sampling"),
cl::cat(BenchmarkOptions), cl::init(0));
+static cl::opt<bool>
+ DisableUpperSSERegisters("x86-disable-upper-sse-registers",
+ cl::desc("Disable XMM8-XMM15 register usage"),
+ cl::cat(BenchmarkOptions), cl::init(false));
+
// FIXME: Validates that repetition-mode is loop if LBR is requested.
// Returns a non-null reason if we cannot handle the memory references in this
@@ -708,6 +713,11 @@ class ExegesisX86Target : public ExegesisTarget {
const APInt &Value) const override;
ArrayRef<unsigned> getUnavailableRegisters() const override {
+ if (DisableUpperSSERegisters)
+ return makeArrayRef(kUnavailableRegistersSSE,
+ sizeof(kUnavailableRegistersSSE) /
+ sizeof(kUnavailableRegistersSSE[0]));
+
return makeArrayRef(kUnavailableRegisters,
std::size(kUnavailableRegisters));
}
@@ -772,6 +782,7 @@ class ExegesisX86Target : public ExegesisTarget {
}
static const unsigned kUnavailableRegisters[4];
+ static const unsigned kUnavailableRegistersSSE[12];
};
// We disable a few registers that cannot be encoded on instructions with a REX
@@ -779,6 +790,12 @@ class ExegesisX86Target : public ExegesisTarget {
const unsigned ExegesisX86Target::kUnavailableRegisters[4] = {X86::AH, X86::BH,
X86::CH, X86::DH};
+// Optionally, also disable the upper (x86_64) SSE registers to reduce frontend
+// decoder load.
+const unsigned ExegesisX86Target::kUnavailableRegistersSSE[12] = {
+ X86::AH, X86::BH, X86::CH, X86::DH, X86::XMM8, X86::XMM9,
+ X86::XMM10, X86::XMM11, X86::XMM12, X86::XMM13, X86::XMM14, X86::XMM15};
+
// We're using one of R8-R15 because these registers are never hardcoded in
// instructions (e.g. MOVS writes to EDI, ESI, EDX), so they have less
// conflicts.
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