+//===- README.txt - Notes for improving PowerPC-specific code gen ---------===//
+
TODO:
-* use stfiwx in float->int
-* implement cast fp to bool
-* implement algebraic shift right long by reg
-* implement scheduling info
-* implement powerpc-64 for darwin
-* implement powerpc-64 for aix
-* fix rlwimi generation to be use-and-def
-* fix ulong to double:
- floatdidf assumes signed longs. so if the high but of a ulong
- just happens to be set, you get the wrong sign. The fix for this
- is to call cmpdi2 to compare against zero, if so shift right by one,
- convert to fp, and multiply by (add to itself). the sequence would
- look like:
- {r3:r4} holds ulong a;
- li r5, 0
- li r6, 0 (set r5:r6 to ulong 0)
- call cmpdi2 ==> sets r3 <, =, > 0
- if r3 > 0
- call floatdidf as usual
- else
- shift right ulong a, 1 (we could use emitShift)
- call floatdidf
- fadd f1, f1, f1 (fp left shift by 1)
-* setCondInst needs to know branchless versions of seteq/setne/etc
-* cast elimination pass (uint -> sbyte -> short, kill the byte -> short)
-* should hint to the branch select pass that it doesn't need to print the
- second unconditional branch, so we don't end up with things like:
- b .LBBl42__2E_expand_function_8_674 ; loopentry.24
- b .LBBl42__2E_expand_function_8_42 ; NewDefault
- b .LBBl42__2E_expand_function_8_42 ; NewDefault
-
-Currently failing tests:
-* SingleSource
- `- Regression
- | `- casts (ulong to fp failure)
- `- Benchmarks
- | `- Shootout-C++ : most programs fail, miscompilations
- `- UnitTests
- | `- C++Catch
- | `- SimpleC++Test
- | `- ConditionalExpr (also C++)
-* MultiSource
- |- Applications
- | `- burg: miscompilation
- | `- hbd: miscompilation
- | `- d (make_dparser): miscompilation
- `- Benchmarks
- `- MallocBench/gs: miscompilation
+* lmw/stmw pass a la arm load store optimizer for prolog/epilog
+
+===-------------------------------------------------------------------------===
+
+This code:
+
+unsigned add32carry(unsigned sum, unsigned x) {
+ unsigned z = sum + x;
+ if (sum + x < x)
+ z++;
+ return z;
+}
+
+Should compile to something like:
+
+ addc r3,r3,r4
+ addze r3,r3
+
+instead we get:
+
+ add r3, r4, r3
+ cmplw cr7, r3, r4
+ mfcr r4 ; 1
+ rlwinm r4, r4, 29, 31, 31
+ add r3, r3, r4
+
+Ick.
+
+===-------------------------------------------------------------------------===
+
+We compile the hottest inner loop of viterbi to:
+
+ li r6, 0
+ b LBB1_84 ;bb432.i
+LBB1_83: ;bb420.i
+ lbzx r8, r5, r7
+ addi r6, r7, 1
+ stbx r8, r4, r7
+LBB1_84: ;bb432.i
+ mr r7, r6
+ cmplwi cr0, r7, 143
+ bne cr0, LBB1_83 ;bb420.i
+
+The CBE manages to produce:
+
+ li r0, 143
+ mtctr r0
+loop:
+ lbzx r2, r2, r11
+ stbx r0, r2, r9
+ addi r2, r2, 1
+ bdz later
+ b loop
+
+This could be much better (bdnz instead of bdz) but it still beats us. If we
+produced this with bdnz, the loop would be a single dispatch group.
+
+===-------------------------------------------------------------------------===
+
+Lump the constant pool for each function into ONE pic object, and reference
+pieces of it as offsets from the start. For functions like this (contrived
+to have lots of constants obviously):
+
+double X(double Y) { return (Y*1.23 + 4.512)*2.34 + 14.38; }
+
+We generate:
+
+_X:
+ lis r2, ha16(.CPI_X_0)
+ lfd f0, lo16(.CPI_X_0)(r2)
+ lis r2, ha16(.CPI_X_1)
+ lfd f2, lo16(.CPI_X_1)(r2)
+ fmadd f0, f1, f0, f2
+ lis r2, ha16(.CPI_X_2)
+ lfd f1, lo16(.CPI_X_2)(r2)
+ lis r2, ha16(.CPI_X_3)
+ lfd f2, lo16(.CPI_X_3)(r2)
+ fmadd f1, f0, f1, f2
+ blr
+
+It would be better to materialize .CPI_X into a register, then use immediates
+off of the register to avoid the lis's. This is even more important in PIC
+mode.
+
+Note that this (and the static variable version) is discussed here for GCC:
+http://gcc.gnu.org/ml/gcc-patches/2006-02/msg00133.html
+
+Here's another example (the sgn function):
+double testf(double a) {
+ return a == 0.0 ? 0.0 : (a > 0.0 ? 1.0 : -1.0);
+}
+
+it produces a BB like this:
+LBB1_1: ; cond_true
+ lis r2, ha16(LCPI1_0)
+ lfs f0, lo16(LCPI1_0)(r2)
+ lis r2, ha16(LCPI1_1)
+ lis r3, ha16(LCPI1_2)
+ lfs f2, lo16(LCPI1_2)(r3)
+ lfs f3, lo16(LCPI1_1)(r2)
+ fsub f0, f0, f1
+ fsel f1, f0, f2, f3
+ blr
+
+===-------------------------------------------------------------------------===
+
+PIC Code Gen IPO optimization:
+
+Squish small scalar globals together into a single global struct, allowing the
+address of the struct to be CSE'd, avoiding PIC accesses (also reduces the size
+of the GOT on targets with one).
+
+Note that this is discussed here for GCC:
+http://gcc.gnu.org/ml/gcc-patches/2006-02/msg00133.html
+
+===-------------------------------------------------------------------------===
+
+Darwin Stub removal:
+
+We still generate calls to foo$stub, and stubs, on Darwin. This is not
+necessary when building with the Leopard (10.5) or later linker, as stubs are
+generated by ld when necessary. Parameterizing this based on the deployment
+target (-mmacosx-version-min) is probably enough. x86-32 does this right, see
+its logic.
+
+===-------------------------------------------------------------------------===
+
+Darwin Stub LICM optimization:
+
+Loops like this:
+
+ for (...) bar();
+
+Have to go through an indirect stub if bar is external or linkonce. It would
+be better to compile it as:
+
+ fp = &bar;
+ for (...) fp();
+
+which only computes the address of bar once (instead of each time through the
+stub). This is Darwin specific and would have to be done in the code generator.
+Probably not a win on x86.
+
+===-------------------------------------------------------------------------===
+
+Simple IPO for argument passing, change:
+ void foo(int X, double Y, int Z) -> void foo(int X, int Z, double Y)
+
+the Darwin ABI specifies that any integer arguments in the first 32 bytes worth
+of arguments get assigned to r3 through r10. That is, if you have a function
+foo(int, double, int) you get r3, f1, r6, since the 64 bit double ate up the
+argument bytes for r4 and r5. The trick then would be to shuffle the argument
+order for functions we can internalize so that the maximum number of
+integers/pointers get passed in regs before you see any of the fp arguments.
+
+Instead of implementing this, it would actually probably be easier to just
+implement a PPC fastcc, where we could do whatever we wanted to the CC,
+including having this work sanely.
+
+===-------------------------------------------------------------------------===
+
+Fix Darwin FP-In-Integer Registers ABI
+
+Darwin passes doubles in structures in integer registers, which is very very
+bad. Add something like a BITCAST to LLVM, then do an i-p transformation that
+percolates these things out of functions.
+
+Check out how horrible this is:
+http://gcc.gnu.org/ml/gcc/2005-10/msg01036.html
+
+This is an extension of "interprocedural CC unmunging" that can't be done with
+just fastcc.
+
+===-------------------------------------------------------------------------===
+
+Fold add and sub with constant into non-extern, non-weak addresses so this:
+
+static int a;
+void bar(int b) { a = b; }
+void foo(unsigned char *c) {
+ *c = a;
+}
+
+So that
+
+_foo:
+ lis r2, ha16(_a)
+ la r2, lo16(_a)(r2)
+ lbz r2, 3(r2)
+ stb r2, 0(r3)
+ blr
+
+Becomes
+
+_foo:
+ lis r2, ha16(_a+3)
+ lbz r2, lo16(_a+3)(r2)
+ stb r2, 0(r3)
+ blr
+
+===-------------------------------------------------------------------------===
+
+We should compile these two functions to the same thing:
+
+#include <stdlib.h>
+void f(int a, int b, int *P) {
+ *P = (a-b)>=0?(a-b):(b-a);
+}
+void g(int a, int b, int *P) {
+ *P = abs(a-b);
+}
+
+Further, they should compile to something better than:
+
+_g:
+ subf r2, r4, r3
+ subfic r3, r2, 0
+ cmpwi cr0, r2, -1
+ bgt cr0, LBB2_2 ; entry
+LBB2_1: ; entry
+ mr r2, r3
+LBB2_2: ; entry
+ stw r2, 0(r5)
+ blr
+
+GCC produces:
+
+_g:
+ subf r4,r4,r3
+ srawi r2,r4,31
+ xor r0,r2,r4
+ subf r0,r2,r0
+ stw r0,0(r5)
+ blr
+
+... which is much nicer.
+
+This theoretically may help improve twolf slightly (used in dimbox.c:142?).
+
+===-------------------------------------------------------------------------===
+
+PR5945: This:
+define i32 @clamp0g(i32 %a) {
+entry:
+ %cmp = icmp slt i32 %a, 0
+ %sel = select i1 %cmp, i32 0, i32 %a
+ ret i32 %sel
+}
+
+Is compile to this with the PowerPC (32-bit) backend:
+
+_clamp0g:
+ cmpwi cr0, r3, 0
+ li r2, 0
+ blt cr0, LBB1_2
+; BB#1: ; %entry
+ mr r2, r3
+LBB1_2: ; %entry
+ mr r3, r2
+ blr
+
+This could be reduced to the much simpler:
+
+_clamp0g:
+ srawi r2, r3, 31
+ andc r3, r3, r2
+ blr
+
+===-------------------------------------------------------------------------===
+
+int foo(int N, int ***W, int **TK, int X) {
+ int t, i;
+
+ for (t = 0; t < N; ++t)
+ for (i = 0; i < 4; ++i)
+ W[t / X][i][t % X] = TK[i][t];
+
+ return 5;
+}
+
+We generate relatively atrocious code for this loop compared to gcc.
+
+We could also strength reduce the rem and the div:
+http://www.lcs.mit.edu/pubs/pdf/MIT-LCS-TM-600.pdf
+
+===-------------------------------------------------------------------------===
+
+We generate ugly code for this:
+
+void func(unsigned int *ret, float dx, float dy, float dz, float dw) {
+ unsigned code = 0;
+ if(dx < -dw) code |= 1;
+ if(dx > dw) code |= 2;
+ if(dy < -dw) code |= 4;
+ if(dy > dw) code |= 8;
+ if(dz < -dw) code |= 16;
+ if(dz > dw) code |= 32;
+ *ret = code;
+}
+
+===-------------------------------------------------------------------------===
+
+%struct.B = type { i8, [3 x i8] }
+
+define void @bar(%struct.B* %b) {
+entry:
+ %tmp = bitcast %struct.B* %b to i32* ; <uint*> [#uses=1]
+ %tmp = load i32* %tmp ; <uint> [#uses=1]
+ %tmp3 = bitcast %struct.B* %b to i32* ; <uint*> [#uses=1]
+ %tmp4 = load i32* %tmp3 ; <uint> [#uses=1]
+ %tmp8 = bitcast %struct.B* %b to i32* ; <uint*> [#uses=2]
+ %tmp9 = load i32* %tmp8 ; <uint> [#uses=1]
+ %tmp4.mask17 = shl i32 %tmp4, i8 1 ; <uint> [#uses=1]
+ %tmp1415 = and i32 %tmp4.mask17, 2147483648 ; <uint> [#uses=1]
+ %tmp.masked = and i32 %tmp, 2147483648 ; <uint> [#uses=1]
+ %tmp11 = or i32 %tmp1415, %tmp.masked ; <uint> [#uses=1]
+ %tmp12 = and i32 %tmp9, 2147483647 ; <uint> [#uses=1]
+ %tmp13 = or i32 %tmp12, %tmp11 ; <uint> [#uses=1]
+ store i32 %tmp13, i32* %tmp8
+ ret void
+}
+
+We emit:
+
+_foo:
+ lwz r2, 0(r3)
+ slwi r4, r2, 1
+ or r4, r4, r2
+ rlwimi r2, r4, 0, 0, 0
+ stw r2, 0(r3)
+ blr
+
+We could collapse a bunch of those ORs and ANDs and generate the following
+equivalent code:
+
+_foo:
+ lwz r2, 0(r3)
+ rlwinm r4, r2, 1, 0, 0
+ or r2, r2, r4
+ stw r2, 0(r3)
+ blr
+
+===-------------------------------------------------------------------------===
+
+Consider a function like this:
+
+float foo(float X) { return X + 1234.4123f; }
+
+The FP constant ends up in the constant pool, so we need to get the LR register.
+ This ends up producing code like this:
+
+_foo:
+.LBB_foo_0: ; entry
+ mflr r11
+*** stw r11, 8(r1)
+ bl "L00000$pb"
+"L00000$pb":
+ mflr r2
+ addis r2, r2, ha16(.CPI_foo_0-"L00000$pb")
+ lfs f0, lo16(.CPI_foo_0-"L00000$pb")(r2)
+ fadds f1, f1, f0
+*** lwz r11, 8(r1)
+ mtlr r11
+ blr
+
+This is functional, but there is no reason to spill the LR register all the way
+to the stack (the two marked instrs): spilling it to a GPR is quite enough.
+
+Implementing this will require some codegen improvements. Nate writes:
+
+"So basically what we need to support the "no stack frame save and restore" is a
+generalization of the LR optimization to "callee-save regs".
+
+Currently, we have LR marked as a callee-save reg. The register allocator sees
+that it's callee save, and spills it directly to the stack.
+
+Ideally, something like this would happen:
+
+LR would be in a separate register class from the GPRs. The class of LR would be
+marked "unspillable". When the register allocator came across an unspillable
+reg, it would ask "what is the best class to copy this into that I *can* spill"
+If it gets a class back, which it will in this case (the gprs), it grabs a free
+register of that class. If it is then later necessary to spill that reg, so be
+it.
+
+===-------------------------------------------------------------------------===
+
+We compile this:
+int test(_Bool X) {
+ return X ? 524288 : 0;
+}
+
+to:
+_test:
+ cmplwi cr0, r3, 0
+ lis r2, 8
+ li r3, 0
+ beq cr0, LBB1_2 ;entry
+LBB1_1: ;entry
+ mr r3, r2
+LBB1_2: ;entry
+ blr
+
+instead of:
+_test:
+ addic r2,r3,-1
+ subfe r0,r2,r3
+ slwi r3,r0,19
+ blr
+
+This sort of thing occurs a lot due to globalopt.
+
+===-------------------------------------------------------------------------===
+
+We compile:
+
+define i32 @bar(i32 %x) nounwind readnone ssp {
+entry:
+ %0 = icmp eq i32 %x, 0 ; <i1> [#uses=1]
+ %neg = sext i1 %0 to i32 ; <i32> [#uses=1]
+ ret i32 %neg
+}
+
+to:
+
+_bar:
+ cntlzw r2, r3
+ slwi r2, r2, 26
+ srawi r3, r2, 31
+ blr
+
+it would be better to produce:
+
+_bar:
+ addic r3,r3,-1
+ subfe r3,r3,r3
+ blr
+
+===-------------------------------------------------------------------------===
+
+We generate horrible ppc code for this:
+
+#define N 2000000
+double a[N],c[N];
+void simpleloop() {
+ int j;
+ for (j=0; j<N; j++)
+ c[j] = a[j];
+}
+
+LBB1_1: ;bb
+ lfdx f0, r3, r4
+ addi r5, r5, 1 ;; Extra IV for the exit value compare.
+ stfdx f0, r2, r4
+ addi r4, r4, 8
+
+ xoris r6, r5, 30 ;; This is due to a large immediate.
+ cmplwi cr0, r6, 33920
+ bne cr0, LBB1_1
+
+//===---------------------------------------------------------------------===//
+
+This:
+ #include <algorithm>
+ inline std::pair<unsigned, bool> full_add(unsigned a, unsigned b)
+ { return std::make_pair(a + b, a + b < a); }
+ bool no_overflow(unsigned a, unsigned b)
+ { return !full_add(a, b).second; }
+
+Should compile to:
+
+__Z11no_overflowjj:
+ add r4,r3,r4
+ subfc r3,r3,r4
+ li r3,0
+ adde r3,r3,r3
+ blr
+
+(or better) not:
+
+__Z11no_overflowjj:
+ add r2, r4, r3
+ cmplw cr7, r2, r3
+ mfcr r2
+ rlwinm r2, r2, 29, 31, 31
+ xori r3, r2, 1
+ blr
+
+//===---------------------------------------------------------------------===//
+
+We compile some FP comparisons into an mfcr with two rlwinms and an or. For
+example:
+#include <math.h>
+int test(double x, double y) { return islessequal(x, y);}
+int test2(double x, double y) { return islessgreater(x, y);}
+int test3(double x, double y) { return !islessequal(x, y);}
+
+Compiles into (all three are similar, but the bits differ):
+
+_test:
+ fcmpu cr7, f1, f2
+ mfcr r2
+ rlwinm r3, r2, 29, 31, 31
+ rlwinm r2, r2, 31, 31, 31
+ or r3, r2, r3
+ blr
+
+GCC compiles this into:
+
+ _test:
+ fcmpu cr7,f1,f2
+ cror 30,28,30
+ mfcr r3
+ rlwinm r3,r3,31,1
+ blr
+
+which is more efficient and can use mfocr. See PR642 for some more context.
+
+//===---------------------------------------------------------------------===//
+
+void foo(float *data, float d) {
+ long i;
+ for (i = 0; i < 8000; i++)
+ data[i] = d;
+}
+void foo2(float *data, float d) {
+ long i;
+ data--;
+ for (i = 0; i < 8000; i++) {
+ data[1] = d;
+ data++;
+ }
+}
+
+These compile to:
+
+_foo:
+ li r2, 0
+LBB1_1: ; bb
+ addi r4, r2, 4
+ stfsx f1, r3, r2
+ cmplwi cr0, r4, 32000
+ mr r2, r4
+ bne cr0, LBB1_1 ; bb
+ blr
+_foo2:
+ li r2, 0
+LBB2_1: ; bb
+ addi r4, r2, 4
+ stfsx f1, r3, r2
+ cmplwi cr0, r4, 32000
+ mr r2, r4
+ bne cr0, LBB2_1 ; bb
+ blr
+
+The 'mr' could be eliminated to folding the add into the cmp better.
+
+//===---------------------------------------------------------------------===//
+Codegen for the following (low-probability) case deteriorated considerably
+when the correctness fixes for unordered comparisons went in (PR 642, 58871).
+It should be possible to recover the code quality described in the comments.
+
+; RUN: llvm-as < %s | llc -march=ppc32 | grep or | count 3
+; This should produce one 'or' or 'cror' instruction per function.
+
+; RUN: llvm-as < %s | llc -march=ppc32 | grep mfcr | count 3
+; PR2964
+
+define i32 @test(double %x, double %y) nounwind {
+entry:
+ %tmp3 = fcmp ole double %x, %y ; <i1> [#uses=1]
+ %tmp345 = zext i1 %tmp3 to i32 ; <i32> [#uses=1]
+ ret i32 %tmp345
+}
+
+define i32 @test2(double %x, double %y) nounwind {
+entry:
+ %tmp3 = fcmp one double %x, %y ; <i1> [#uses=1]
+ %tmp345 = zext i1 %tmp3 to i32 ; <i32> [#uses=1]
+ ret i32 %tmp345
+}
+
+define i32 @test3(double %x, double %y) nounwind {
+entry:
+ %tmp3 = fcmp ugt double %x, %y ; <i1> [#uses=1]
+ %tmp34 = zext i1 %tmp3 to i32 ; <i32> [#uses=1]
+ ret i32 %tmp34
+}
+//===----------------------------------------------------------------------===//
+; RUN: llvm-as < %s | llc -march=ppc32 | not grep fneg
+
+; This could generate FSEL with appropriate flags (FSEL is not IEEE-safe, and
+; should not be generated except with -enable-finite-only-fp-math or the like).
+; With the correctness fixes for PR642 (58871) LowerSELECT_CC would need to
+; recognize a more elaborate tree than a simple SETxx.
+
+define double @test_FNEG_sel(double %A, double %B, double %C) {
+ %D = fsub double -0.000000e+00, %A ; <double> [#uses=1]
+ %Cond = fcmp ugt double %D, -0.000000e+00 ; <i1> [#uses=1]
+ %E = select i1 %Cond, double %B, double %C ; <double> [#uses=1]
+ ret double %E
+}
+
+//===----------------------------------------------------------------------===//
+The save/restore sequence for CR in prolog/epilog is terrible:
+- Each CR subreg is saved individually, rather than doing one save as a unit.
+- On Darwin, the save is done after the decrement of SP, which means the offset
+from SP of the save slot can be too big for a store instruction, which means we
+need an additional register (currently hacked in 96015+96020; the solution there
+is correct, but poor).
+- On SVR4 the same thing can happen, and I don't think saving before the SP
+decrement is safe on that target, as there is no red zone. This is currently
+broken AFAIK, although it's not a target I can exercise.
+The following demonstrates the problem:
+extern void bar(char *p);
+void foo() {
+ char x[100000];
+ bar(x);
+ __asm__("" ::: "cr2");
+}
+
+//===-------------------------------------------------------------------------===
+Naming convention for instruction formats is very haphazard.
+We have agreed on a naming scheme as follows:
+
+<INST_form>{_<OP_type><OP_len>}+
+
+Where:
+INST_form is the instruction format (X-form, etc.)
+OP_type is the operand type - one of OPC (opcode), RD (register destination),
+ RS (register source),
+ RDp (destination register pair),
+ RSp (source register pair), IM (immediate),
+ XO (extended opcode)
+OP_len is the length of the operand in bits
+
+VSX register operands would be of length 6 (split across two fields),
+condition register fields of length 3.
+We would not need denote reserved fields in names of instruction formats.
+
+//===----------------------------------------------------------------------===//
+
+Instruction fusion was introduced in ISA 2.06 and more opportunities added in
+ISA 2.07. LLVM needs to add infrastructure to recognize fusion opportunities
+and force instruction pairs to be scheduled together.
+
projects / oota-llvm.git / blobdiff