[Git][ghc/ghc][wip/andreask/fma_x86] Fix FMA primops generating broken assembly on x86.

[Andreas Klebinger (@AndreasK)]

Andreas Klebinger pushed to branch wip/andreask/fma_x86 at Glasgow Haskell Compiler / GHC


Commits:
3b259e17 by Andreas Klebinger at 2023-11-17T12:56:43+01:00
Fix FMA primops generating broken assembly on x86.

`genFMA3Code` assumed that we had to take extra precations to avoid overwriting
the result of `getNonClobberedReg`. One of these special cases caused a bug resulting
in broken assembly.

I believe we don't need to hadle these cases specially at all, which means this MR simply
deletes the special cases to fix the bug.

Fixes #24160

- - - - -


4 changed files:

- compiler/GHC/CmmToAsm/X86/CodeGen.hs
- compiler/GHC/CmmToAsm/X86/Instr.hs
- + testsuite/tests/primops/should_compile/T24160_FMA.hs
- testsuite/tests/primops/should_compile/all.T


Changes:

=====================================
compiler/GHC/CmmToAsm/X86/CodeGen.hs
=====================================
@@ -3448,32 +3448,9 @@ genFMA3Code w signs x y z = do
   (y_reg, y_code) <- getNonClobberedReg y
   (z_reg, z_code) <- getNonClobberedReg z
   x_code <- getAnyReg x
-  y_tmp <- getNewRegNat rep
-  z_tmp <- getNewRegNat rep
   let
      fma213 = FMA3 rep signs FMA213
      code dst
-         | dst == y_reg
-         , dst == z_reg
-         = y_code `appOL`
-           unitOL (MOV rep (OpReg y_reg) (OpReg y_tmp)) `appOL`
-           z_code `appOL`
-           unitOL (MOV rep (OpReg z_reg) (OpReg z_tmp)) `appOL`
-           x_code dst `snocOL`
-           fma213 (OpReg z_tmp) y_tmp dst
-        | dst == y_reg
-        = y_code `appOL`
-          unitOL (MOV rep (OpReg y_reg) (OpReg z_tmp)) `appOL`
-          z_code `appOL`
-          x_code dst `snocOL`
-          fma213 (OpReg z_reg) y_tmp dst
-        | dst == z_reg
-        = y_code `appOL`
-          z_code `appOL`
-          unitOL (MOV rep (OpReg z_reg) (OpReg z_tmp)) `appOL`
-          x_code dst `snocOL`
-          fma213 (OpReg z_tmp) y_reg dst
-        | otherwise
         = y_code `appOL`
           z_code `appOL`
           x_code dst `snocOL`


=====================================
compiler/GHC/CmmToAsm/X86/Instr.hs
=====================================
@@ -277,7 +277,8 @@ data Instr
 
         -- | FMA3 fused multiply-add operations.
         | FMA3         Format FMASign FMAPermutation Operand Reg Reg
-          -- src1 (r/m), src2 (r), dst (r)
+          -- src3 (r/m), src2 (r), dst/src1 (r)
+          -- The is exactly reversed from how intel lists the arguments.
 
         -- use ADD, SUB, and SQRT for arithmetic.  In both cases, operands
         -- are  Operand Reg.
@@ -358,6 +359,7 @@ data Operand
         | OpImm  Imm            -- immediate value
         | OpAddr AddrMode       -- memory reference
 
+-- NB: As of 2023 we only use the FMA213 permutation.
 data FMAPermutation = FMA132 | FMA213 | FMA231
 
 -- | Returns which registers are read and written as a (read, written)
@@ -447,7 +449,7 @@ regUsageOfInstr platform instr
     PDEP   _ src mask dst -> mkRU (use_R src $ use_R mask []) [dst]
     PEXT   _ src mask dst -> mkRU (use_R src $ use_R mask []) [dst]
 
-    FMA3 _ _ _ src1 src2 dst -> usageFMA src1 src2 dst
+    FMA3 _ _ _ src3 src2 dst -> usageFMA src3 src2 dst
 
     -- note: might be a better way to do this
     PREFETCH _  _ src -> mkRU (use_R src []) []


=====================================
testsuite/tests/primops/should_compile/T24160_FMA.hs
=====================================
@@ -0,0 +1,8 @@
+{-# LANGUAGE MagicHash #-}
+{-# LANGUAGE UnboxedTuples #-}
+module TwoProdFMA where
+import GHC.Exts
+
+twoProductFloat# :: Float# -> Float# -> (# Float#, Float# #)
+twoProductFloat# x y = let !r = x `timesFloat#` y
+                       in (# r, fmsubFloat# x y r #)


=====================================
testsuite/tests/primops/should_compile/all.T
=====================================
@@ -8,3 +8,8 @@ test('UnliftedStableName', normal, compile, [''])
 test('KeepAliveWrapper', normal, compile, ['-O'])
 test('T22152', normal, compile, [''])
 test('T22152b', normal, compile, [''])
+test('T24160_FMA'
+    , [ js_skip # JS backend doesn't have an FMA implementation ]
+      # , omit_ghci # fails during compilation phase, remove after !10563
+      ]
+    , compile, ['-O'])
\ No newline at end of file



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/3b259e17b6aface99be80a34c17e7a2c92fc89b0

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/3b259e17b6aface99be80a34c17e7a2c92fc89b0
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