| Message ID | 005a01d91e01$80bd4af0$8237e0d0$@nextmovesoftware.com |
|---|---|
| State | New |
| Headers | show |
| Series | [x86] PR target/108229: A minor STV compute_convert_gain tweak. | expand |
On Sun, Jan 1, 2023 at 5:53 PM Roger Sayle <roger@nextmovesoftware.com> wrote: > > > This patch addresses PR target/108229, which is a change in code > generation during the STV pass, due to the recently approved patch > to handle vec_select (reductions) in the vector unit. The recent > change is innocent, but exposes a latent STV "gain" calculation issue > that is benign (or closely balanced) on most microarchitectures. > > The issue is when STV considers converting PLUS with a MEM operand. > > On TARGET_64BIT (m=1): > addq 24(%rdi), %rdx // 4 bytes > or with -m32 (m=2) > addl 24(%esi), %eax // 3 bytes > adcl 28(%esi), %edx // 3 bytes > is being converted by STV to > vmovq 24(%rdi), %xmm5 // 5 bytes > vpaddq %xmm5, %xmm4, %xmm4 // 4 bytes > > The current code in general_scalar_chain::compute_convert_gain > considers that scalar unit addition is replaced with a vector > unit addition (usually about the same cost), but doesn't consider > anything special about MEM operands, assuming that a scalar load > gains/costs nothing compared to a vector load. We can allow the > backend slightly better fine tuning by including in the gain > calculation that m scalar loads are being replaced by one vector > load, and when optimizing for size including that we're increasing > code size (e.g. an extra vmovq instruction for a MEM operand). > > This patch is a win on the CSiBE benchmark when compiled with -Os. > Alas I've no new testcase as this is extremely sensitive to the > backend microarchitecture parameterization (and it's dangerous to > select parameters from the N=1 statistics of a single bugzilla PR). > > This patch has been tested on x86_64-pc-linux-gnu with make bootstrap > and make -k check, both with and without --target_board=unix{-m32}, > with no new failures. Ok for mainline? > > > 2023-01-01 Roger Sayle <roger@nextmovesoftware.com> > > gcc/ChangeLog > PR target/108229 > * config/i386/i386-features.cc > (general_scalar_chain::compute_convert_gain) <case PLUS>: Consider > the gain/cost of converting a MEM operand. LGTM. Thanks, Uros. > > Thanks in advance, > Roger > -- >
diff --git a/gcc/config/i386/i386-features.cc b/gcc/config/i386/i386-features.cc index 0f59be0..f420f83 100644 --- a/gcc/config/i386/i386-features.cc +++ b/gcc/config/i386/i386-features.cc @@ -567,6 +567,14 @@ general_scalar_chain::compute_convert_gain () igain -= vector_const_cost (XEXP (src, 0)); if (CONST_INT_P (XEXP (src, 1))) igain -= vector_const_cost (XEXP (src, 1)); + if (MEM_P (XEXP (src, 1))) + { + if (optimize_insn_for_size_p ()) + igain -= COSTS_N_BYTES (m == 2 ? 3 : 5); + else + igain += m * ix86_cost->int_load[2] + - ix86_cost->sse_load[sse_cost_idx]; + } break; case NEG:
This patch addresses PR target/108229, which is a change in code generation during the STV pass, due to the recently approved patch to handle vec_select (reductions) in the vector unit. The recent change is innocent, but exposes a latent STV "gain" calculation issue that is benign (or closely balanced) on most microarchitectures. The issue is when STV considers converting PLUS with a MEM operand. On TARGET_64BIT (m=1): addq 24(%rdi), %rdx // 4 bytes or with -m32 (m=2) addl 24(%esi), %eax // 3 bytes adcl 28(%esi), %edx // 3 bytes is being converted by STV to vmovq 24(%rdi), %xmm5 // 5 bytes vpaddq %xmm5, %xmm4, %xmm4 // 4 bytes The current code in general_scalar_chain::compute_convert_gain considers that scalar unit addition is replaced with a vector unit addition (usually about the same cost), but doesn't consider anything special about MEM operands, assuming that a scalar load gains/costs nothing compared to a vector load. We can allow the backend slightly better fine tuning by including in the gain calculation that m scalar loads are being replaced by one vector load, and when optimizing for size including that we're increasing code size (e.g. an extra vmovq instruction for a MEM operand). This patch is a win on the CSiBE benchmark when compiled with -Os. Alas I've no new testcase as this is extremely sensitive to the backend microarchitecture parameterization (and it's dangerous to select parameters from the N=1 statistics of a single bugzilla PR). This patch has been tested on x86_64-pc-linux-gnu with make bootstrap and make -k check, both with and without --target_board=unix{-m32}, with no new failures. Ok for mainline? 2023-01-01 Roger Sayle <roger@nextmovesoftware.com> gcc/ChangeLog PR target/108229 * config/i386/i386-features.cc (general_scalar_chain::compute_convert_gain) <case PLUS>: Consider the gain/cost of converting a MEM operand. Thanks in advance, Roger --