Mesa (master): glsl: Fix software 64-bit integer to 32-bit float conversions.

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Module: Mesa
Branch: master
Commit: a30bb25a7a495db7b7cb3be50431029f48019fc3
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=a30bb25a7a495db7b7cb3be50431029f48019fc3

Author: Francisco Jerez <currojerez at riseup.net>
Date:   Fri Dec 27 14:10:31 2019 -0800

glsl: Fix software 64-bit integer to 32-bit float conversions.

The current implementation was broken for any integers between 2^24
and 2^30 (it would return zero for me on ICL).  The reason is that for
such integers we wouldn't take the 'if (0 <= shiftCount)' early return
path, however 'shiftCount + 7' would be positive, leading to a
negative 'count' argument passed to __shift64RightJamming(), which
would give undefined results.

This reworks the affected conversion functions to use either
__shortShift64Left() or __shift64RightJamming() based on the sign of
the final shift count, which should avoid the problem.  In addition
this should qualify as a clean-up/optimization -- This implementation
of the conversion functions translates to 7 instructions less than the
original on Intel hardware.

This fixes the 'KHR-GL46.shader_ballot_tests.ShaderBallotFunctionBallot'
conformance tests on soft fp64 hardware with large enough subgroup
size (>16).

Fixes: d5cf6e92b4f7 "glsl: Add built-in functions to do uint64_to_fp32(uint64_t)"
Fixes: c9d333a6b76e "glsl: Add built-in functions to do int64_to_fp32(int64_t)"
Cc: Sagar Ghuge <sagar.ghuge at intel.com>
Reviewed-by: Sagar Ghuge <sagar.ghuge at intel.com>

---

 src/compiler/glsl/float64.glsl | 36 ++++++++++++++----------------------
 1 file changed, 14 insertions(+), 22 deletions(-)

diff --git a/src/compiler/glsl/float64.glsl b/src/compiler/glsl/float64.glsl
index 0433d925a39..5f201c8f725 100644
--- a/src/compiler/glsl/float64.glsl
+++ b/src/compiler/glsl/float64.glsl
@@ -1299,43 +1299,35 @@ __fp64_to_fp32(uint64_t __a)
 float
 __uint64_to_fp32(uint64_t __a)
 {
-   uint zFrac = 0u;
    uvec2 aFrac = unpackUint2x32(__a);
-   int shiftCount = __countLeadingZeros32(mix(aFrac.y, aFrac.x, aFrac.y == 0u));
-   shiftCount -= mix(40, 8, aFrac.y == 0u);
+   int shiftCount = mix(__countLeadingZeros32(aFrac.y) - 33,
+                        __countLeadingZeros32(aFrac.x) - 1,
+                        aFrac.y == 0u);
 
-   if (0 <= shiftCount) {
+   if (0 <= shiftCount)
       __shortShift64Left(aFrac.y, aFrac.x, shiftCount, aFrac.y, aFrac.x);
-      bool is_zero = (aFrac.y | aFrac.x) == 0u;
-      return mix(__packFloat32(0u, 0x95 - shiftCount, aFrac.x), 0, is_zero);
-   }
+   else
+      __shift64RightJamming(aFrac.y, aFrac.x, -shiftCount, aFrac.y, aFrac.x);
 
-   shiftCount += 7;
-   __shift64RightJamming(aFrac.y, aFrac.x, -shiftCount, aFrac.y, aFrac.x);
-   zFrac = mix(aFrac.x<<shiftCount, aFrac.x, shiftCount < 0);
-   return __roundAndPackFloat32(0u, 0x9C - shiftCount, zFrac);
+   return __roundAndPackFloat32(0u, 0x9C - shiftCount, aFrac.x);
 }
 
 float
 __int64_to_fp32(int64_t __a)
 {
-   uint zFrac = 0u;
    uint aSign = uint(__a < 0);
    uint64_t absA = mix(uint64_t(__a), uint64_t(-__a), __a < 0);
    uvec2 aFrac = unpackUint2x32(absA);
-   int shiftCount = __countLeadingZeros32(mix(aFrac.y, aFrac.x, aFrac.y == 0u));
-   shiftCount -= mix(40, 8, aFrac.y == 0u);
+   int shiftCount = mix(__countLeadingZeros32(aFrac.y) - 33,
+                        __countLeadingZeros32(aFrac.x) - 1,
+                        aFrac.y == 0u);
 
-   if (0 <= shiftCount) {
+   if (0 <= shiftCount)
       __shortShift64Left(aFrac.y, aFrac.x, shiftCount, aFrac.y, aFrac.x);
-      bool is_zero = (aFrac.y | aFrac.x) == 0u;
-      return mix(__packFloat32(aSign, 0x95 - shiftCount, aFrac.x), 0, absA == 0u);
-   }
+   else
+      __shift64RightJamming(aFrac.y, aFrac.x, -shiftCount, aFrac.y, aFrac.x);
 
-   shiftCount += 7;
-   __shift64RightJamming(aFrac.y, aFrac.x, -shiftCount, aFrac.y, aFrac.x);
-   zFrac = mix(aFrac.x<<shiftCount, aFrac.x, shiftCount < 0);
-   return __roundAndPackFloat32(aSign, 0x9C - shiftCount, zFrac);
+   return __roundAndPackFloat32(aSign, 0x9C - shiftCount, aFrac.x);
 }
 
 /* Returns the result of converting the single-precision floating-point value