This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

[04/32] [x86] Robustify vzeroupper handling across calls

One of the effects of the function_abi series is to make -fipa-ra
work for partially call-clobbered registers.  E.g. if a call preserves
only the low 32 bits of a register R, we handled the partial clobber
separately from -fipa-ra, and so treated the upper bits of R as
clobbered even if we knew that the target function doesn't touch R.

"Fixing" this caused problems for the vzeroupper handling on x86.
The pass that inserts the vzerouppers assumes that no 256-bit or 512-bit
values are live across a call unless the call takes a 256-bit or 512-bit

      /* Needed mode is set to AVX_U128_CLEAN if there are
	 no 256bit or 512bit modes used in function arguments. */

This implicitly relies on:

   saves SSE registers across calls is Win64 (thus no need to check the
   current ABI here), and with AVX enabled Win64 only guarantees that
   the low 16 bytes are saved.  */

static bool
ix86_hard_regno_call_part_clobbered (rtx_insn *insn ATTRIBUTE_UNUSED,
				     unsigned int regno, machine_mode mode)
  return SSE_REGNO_P (regno) && GET_MODE_SIZE (mode) > 16;

The comment suggests that this code is only needed for Win64 and that
not testing for Win64 is just a simplification.  But in practice it was
needed for correctness on GNU/Linux and other targets too, since without
it the RA would be able to keep 256-bit and 512-bit values in SSE
registers across calls that are known not to clobber them.

This patch conservatively treats calls as AVX_U128_ANY if the RA can see
that some SSE registers are not touched by a call.  There are then no
regressions if the ix86_hard_regno_call_part_clobbered check is disabled
for GNU/Linux (not something we should do, was just for testing).

If in fact we want -fipa-ra to pretend that all functions clobber
SSE registers above 128 bits, it'd certainly be possible to arrange
that.  But IMO that would be an optimisation decision, whereas what
the patch is fixing is a correctness decision.  So I think we should
have this check even so.

2019-09-11  Richard Sandiford  <>

	* config/i386/i386.c: Include function-abi.h.
	(ix86_avx_u128_mode_needed): Treat function calls as AVX_U128_ANY
	if they preserve some 256-bit or 512-bit SSE registers.

Index: gcc/config/i386/i386.c
--- gcc/config/i386/i386.c	2019-09-10 19:56:55.601105594 +0100
+++ gcc/config/i386/i386.c	2019-09-11 19:47:28.506233865 +0100
@@ -95,6 +95,7 @@ #define IN_TARGET_CODE 1
 #include "i386-builtins.h"
 #include "i386-expand.h"
 #include "i386-features.h"
+#include "function-abi.h"
 /* This file should be included last.  */
 #include "target-def.h"
@@ -13511,6 +13512,15 @@ ix86_avx_u128_mode_needed (rtx_insn *ins
+      /* If the function is known to preserve some SSE registers,
+	 RA and previous passes can legitimately rely on that for
+	 modes wider than 256 bits.  It's only safe to issue a
+	 vzeroupper if all SSE registers are clobbered.  */
+      const function_abi &abi = call_insn_abi (insn);
+      if (!hard_reg_set_subset_p (reg_class_contents[ALL_SSE_REGS],
+				  abi.mode_clobbers (V4DImode)))
+	return AVX_U128_ANY;
       return AVX_U128_CLEAN;

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]