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164 lines
6.2 KiB
164 lines
6.2 KiB
8 months ago
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PR target/110792: Early clobber issues with rot32di2_doubleword on i386.
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This patch is a conservative fix for PR target/110792, a wrong-code
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regression affecting doubleword rotations by BITS_PER_WORD, which
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effectively swaps the highpart and lowpart words, when the source to be
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rotated resides in memory. The issue is that if the register used to
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hold the lowpart of the destination is mentioned in the address of
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the memory operand, the current define_insn_and_split unintentionally
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clobbers it before reading the highpart.
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Hence, for the testcase, the incorrectly generated code looks like:
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salq $4, %rdi // calculate address
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movq WHIRL_S+8(%rdi), %rdi // accidentally clobber addr
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movq WHIRL_S(%rdi), %rbp // load (wrong) lowpart
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Traditionally, the textbook way to fix this would be to add an
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explicit early clobber to the instruction's constraints.
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(define_insn_and_split "<insn>32di2_doubleword"
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- [(set (match_operand:DI 0 "register_operand" "=r,r,r")
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+ [(set (match_operand:DI 0 "register_operand" "=r,r,&r")
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(any_rotate:DI (match_operand:DI 1 "nonimmediate_operand" "0,r,o")
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(const_int 32)))]
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but unfortunately this currently generates significantly worse code,
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due to a strange choice of reloads (effectively memcpy), which ends up
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looking like:
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salq $4, %rdi // calculate address
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movdqa WHIRL_S(%rdi), %xmm0 // load the double word in SSE reg.
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movaps %xmm0, -16(%rsp) // store the SSE reg back to the stack
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movq -8(%rsp), %rdi // load highpart
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movq -16(%rsp), %rbp // load lowpart
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Note that reload's "&" doesn't distinguish between the memory being
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early clobbered, vs the registers used in an addressing mode being
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early clobbered.
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The fix proposed in this patch is to remove the third alternative, that
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allowed offsetable memory as an operand, forcing reload to place the
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operand into a register before the rotation. This results in:
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salq $4, %rdi
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movq WHIRL_S(%rdi), %rax
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movq WHIRL_S+8(%rdi), %rdi
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movq %rax, %rbp
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I believe there's a more advanced solution, by swapping the order of
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the loads (if first destination register is mentioned in the address),
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or inserting a lea insn (if both destination registers are mentioned
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in the address), but this fix is a minimal "safe" solution, that
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should hopefully be suitable for backporting.
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2023-08-06 Roger Sayle <roger@nextmovesoftware.com>
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gcc/testsuite/ChangeLog
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PR target/110792
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* gcc.target/i386/pr110792.c: Remove dg-final scan-assembler-not.
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2023-08-03 Roger Sayle <roger@nextmovesoftware.com>
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gcc/ChangeLog
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PR target/110792
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* config/i386/i386.md (<any_rotate>ti3): For rotations by 64 bits
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place operand in a register before gen_<insn>64ti2_doubleword.
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(<any_rotate>di3): Likewise, for rotations by 32 bits, place
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operand in a register before gen_<insn>32di2_doubleword.
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(<any_rotate>32di2_doubleword): Constrain operand to be in register.
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(<any_rotate>64ti2_doubleword): Likewise.
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gcc/testsuite/ChangeLog
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PR target/110792
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* g++.target/i386/pr110792.C: New 32-bit C++ test case.
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* gcc.target/i386/pr110792.c: New 64-bit C test case.
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--- gcc/config/i386/i386.md
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+++ gcc/config/i386/i386.md
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@@ -15341,7 +15341,10 @@ (define_expand "<insn>ti3"
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emit_insn (gen_ix86_<insn>ti3_doubleword
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(operands[0], operands[1], operands[2]));
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else if (CONST_INT_P (operands[2]) && INTVAL (operands[2]) == 64)
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- emit_insn (gen_<insn>64ti2_doubleword (operands[0], operands[1]));
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+ {
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+ operands[1] = force_reg (TImode, operands[1]);
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+ emit_insn (gen_<insn>64ti2_doubleword (operands[0], operands[1]));
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+ }
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else
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{
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rtx amount = force_reg (QImode, operands[2]);
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@@ -15376,7 +15379,10 @@ (define_expand "<insn>di3"
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emit_insn (gen_ix86_<insn>di3_doubleword
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(operands[0], operands[1], operands[2]));
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else if (CONST_INT_P (operands[2]) && INTVAL (operands[2]) == 32)
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- emit_insn (gen_<insn>32di2_doubleword (operands[0], operands[1]));
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+ {
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+ operands[1] = force_reg (DImode, operands[1]);
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+ emit_insn (gen_<insn>32di2_doubleword (operands[0], operands[1]));
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+ }
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else
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FAIL;
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@@ -15544,8 +15550,8 @@ (define_insn_and_split "ix86_rotr<dwi>3_doubleword"
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})
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(define_insn_and_split "<insn>32di2_doubleword"
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- [(set (match_operand:DI 0 "register_operand" "=r,r,r")
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- (any_rotate:DI (match_operand:DI 1 "nonimmediate_operand" "0,r,o")
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+ [(set (match_operand:DI 0 "register_operand" "=r,r")
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+ (any_rotate:DI (match_operand:DI 1 "register_operand" "0,r")
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(const_int 32)))]
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"!TARGET_64BIT"
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"#"
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@@ -15562,8 +15568,8 @@ (define_insn_and_split "<insn>32di2_doubleword"
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})
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(define_insn_and_split "<insn>64ti2_doubleword"
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- [(set (match_operand:TI 0 "register_operand" "=r,r,r")
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- (any_rotate:TI (match_operand:TI 1 "nonimmediate_operand" "0,r,o")
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+ [(set (match_operand:TI 0 "register_operand" "=r,r")
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+ (any_rotate:TI (match_operand:TI 1 "register_operand" "0,r")
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(const_int 64)))]
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"TARGET_64BIT"
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"#"
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--- gcc/testsuite/g++.target/i386/pr110792.C
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+++ gcc/testsuite/g++.target/i386/pr110792.C
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@@ -0,0 +1,16 @@
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+/* { dg-do compile { target ia32 } } */
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+/* { dg-options "-O2" } */
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+
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+template <int ROT, typename T>
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+inline T rotr(T input)
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+{
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+ return static_cast<T>((input >> ROT) | (input << (8 * sizeof(T) - ROT)));
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+}
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+
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+unsigned long long WHIRL_S[256] = {0x18186018C07830D8};
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+unsigned long long whirl(unsigned char x0)
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+{
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+ const unsigned long long s4 = WHIRL_S[x0&0xFF];
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+ return rotr<32>(s4);
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+}
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+/* { dg-final { scan-assembler-not "movl\tWHIRL_S\\+4\\(,%eax,8\\), %eax" } } */
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--- gcc/testsuite/gcc.target/i386/pr110792.c
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+++ gcc/testsuite/gcc.target/i386/pr110792.c
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@@ -0,0 +1,17 @@
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+/* { dg-do compile { target int128 } } */
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+/* { dg-options "-O2" } */
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+
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+static inline unsigned __int128 rotr(unsigned __int128 input)
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+{
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+ return ((input >> 64) | (input << (64)));
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+}
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+
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+unsigned __int128 WHIRL_S[256] = {((__int128)0x18186018C07830D8) << 64 |0x18186018C07830D8};
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+unsigned __int128 whirl(unsigned char x0)
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+{
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+ register int t __asm("rdi") = x0&0xFF;
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+ const unsigned __int128 s4 = WHIRL_S[t];
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+ register unsigned __int128 tt __asm("rdi") = rotr(s4);
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+ asm("":::"memory");
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+ return tt;
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+}
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